Just brainstorming here ... specifics of each house and parking spot will blow my generalizations out of the water and will be pertinent in EVERY case, but sort of generally speaking:
I sort of think the "sweet spot" for this new slavable wall connector is either <=100A or >=200A, with the range >100A and <200A being the less-than-sweet spot. (Fair warning: maybe I'm completely wrong because I haven't fully thought this through, and installers and owners will quickly make my brainstorming about the "sweet spot" ranges moot and useless.) While mulling that over just now, I thought one simplistic perhaps dumb perhaps sometimes sub-optimal way to approach that would be to round down or up to the nearest, and behave that way, such that if you have 120A, just slave them for a 100A and throw away the extra 20A capacity, or if you have 180A, just set them both up as 90A (or whatever is just under that) and forgo the slaving. If you are closer to the middle of >100A <200A, then you have to figure out your most stringent use patterns. Hopefully, this will be a house with a commute under 1 hour, and it will hardly matter, since that is almost never a problem, but you kind of have to take your worst days of the month as the design cases. Like, if I had 150A available, and my commute was 2 hours, then what would I do for two cars? For 3 Tesla home, that's easy: master+slave both share 100A, and dedicate 50A to one more. But for only 2 Tesla home? I'd lean toward independent 100A and 50A, and the person who gets home first always gets on the 50A so the late comer has ability to get ready for work faster.
Ok, I think I answered my own questions. 120A -- just round down to 100A and let the slaving work its charms (usually easiest option). Anything above, and dedicate 100A to one (or multiple slavable units if >2 Teslas per home/place), and the rest (total minus 100A) to another wall connector. Whoever gets there first gets the slowest charger (unless they got home really early for shower and are going on errands right away, to free up that fast charger spot). Alternatively, if they get home first and people in the household get along and have each others' keys, the first one can fast charge and then the second one can unplug them (or move them to the slow charger) and take over in the fast charge spot. If 3 Teslas in a 150A home with one 100A circuit with two slavable wall connectors and one 50A circuit with one wall connector, first person home gets on the 50A, and the rest of the people get on the 100A. This can be adjusted for the first person who gets home super early getting on the 100A IF and only if they get off it quickly (to do errands), leaving the 100A for the late comers. Possibly, the 100A can be the go-to for everyone all the time, but every evening, anybody plugged into the 100A if not everyone is home yet has to go unplug. "Oh, he's coming home late again -- unplug!" You'd have to have a spare key dongle for the 100A charger for the early comers so the late comer can unplug their cars.
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Future design wishes from Tesla: In a more ideal situation, a new upgraded Tesla wall connector could be designed and released that could be programmed with a different maximum total load draw for all connectors than the independent load draws. This would be a specialized master controller that would have an additional setting knob which set the "total maximum load" to above 100A (it could literally be anything up to the number of slaved units times 100A), and specialized slaves that can be set to their respective maximum breaker loads (even if 100A was the assumption, that wouldn't be the end of the world since we're already talking above 100A totals here, but it would be nice if some circuits could be lower than 100A for max flexibility). If they ever software-unlimited the slave bus up to, say, 16 units, and had a mother-honking transformer with breaker box rated at 1200 amps (!!!), then that max load would be 1200 amps, each wall connector connected via 100A circuits and breakers to that load panel. Or more realistically, a 600 amp service panel with max load of, say, 300 amps available for charging Teslas, and a 4 Tesla household (mom X, dad S, daughter and son each m3), then each wall connector would have a 100 amp breaker and the master would be set for 100 amps max power for itself and 300 amps max power for all 4 wall connectors.
Come to think of it (I already back-edited this thought into the above paragraph), each slave would need to know its specific breaker amperage -- so there would have to be dual settings on the slaves, as well: one setting for the breaker for that slave (could be anything, not just 100 amp), and another for the setting that it must act as a slave. (As I already said, an artificial restriction that max-total-load-above-100A systems all have 100A independent circuits would be suboptimal but acceptable since the panel would supposedly have enough for 100A and you would just have more stringent requirements for conduit and wire. Preferably, each unit could specify its actual amperage as other than 100A still.) Since the master would require two knobs (one for max amps and one for amps on that particular unit), the slaves would also do the same thing, one knob for amps on that unit and one that says the max amps = I'm a slave and let the master figure it out. With only 4 total in a set today, that's up to 380 amps available to split (>=400amps you don't need to have any slaves), and quite a bit more data bandwidth that can fit in that "unused dip switch" I saw in the manual, but I suppose one could get really crafty and find a subset of good setups for that, such as "if dip is on, knob has settings for I'm a slave on a circuit of 20A, 30A, 40A, 50A, 60A, 70A, 80A, 90A, 100A, or I'm a master and total amps is 125A, 150A, 175A, 200A, 225A, 250A, 275A, 300A, 350A." That's two more slots than are in the bandwidth, so an electrician should pull out the least common, perhaps adjust for the most common use cases -- idk, like nix the 20A slave setting and the 275A total setting. At least that last bit gives that extra flexibility; if this is software controlled, I see absolutely no reason the current units couldn't be firmware upgraded for exactly this ability, and it would make these wall connectors wildly more flexible (although perhaps slightly more complicated to configure, but electricians are good with that, and I conjecture MUCH easier to provision for the use case of homes with above 100A available power where available power is less than 100A * number of wall connectors, I suspect an extremely large percentage of use cases).
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Before anybody complains that having 2 or more Teslas is not a problem anybody ought to have consternation about (5 fully loaded Model X's would be almost the price of a house here in California), I should remind you that most electrical installations are fairly expensive, last many decades, and will have to consider a use time when Teslas are old and cheap and commonplace well within the lifetimes of the home electrical setup -- when you could go pick up a used Tesla for way less than $10K and have AT LEAST 4 in the parking lot, and you WILL have a HPWC for every one of them, probably installed more than 8 years earlier.
By then, everyone will use Uber? We just don't know! (I've never been able to order Uber on my phone, since the app crashes or gives errors every time, and I've heard nothing but horror stories about Uber, such as slow drivers, drivers that want to talk, or drivers that get lost or want to listen to music, or have a crappy car, but driverless would fix all that.)
