I have extensive recent experience, having driven a 5,000 mile not direct route from Florida to SoCal between Aug. 28 and Sep. 14.
5,009 Miles from Florida to SoCal via Lake Superior
Basically, for optimum speed, never charge more than 70% when using a Supercharger on a long trip.
That's a good rule, with the following evidence from two cross country drives this summer to back it up.
I observed that on an Interstate trip it's rarely necessary to charge past 225 rated miles. That's just over 70% on a Model 3LR but closer to 60% on a Raven Model S. With charge speed being good up to 225 rated miles, I may consider skipping a charger if I don't have to drive more than around 130 miles to the next one. Also, if you research chargers online before your trip, you may find that some are out of the way or have minimal facilities nearby, so you may want to arrange skipping those.
The most annoying out-of-the-way charger is St. George, UT, about a 15 minute detour from I-15. Worse, both southbound visits this year have been on weekends with 25 minute waits to get a stall. Northbound I'm staying overnight with friends in Las Vegas and so can leave with close to a full charge and drive direct to Beaver. But that consumes 299 rated miles while limiting TACC to 80mph (Utah's speed limit).
I think aiming for 20% arrival is too high. The reason is you can somewhat control your burn. If you drive 80mph and plan to arrive at 10%... say extra AC has that arrival dropping, slow down to 75 or 70. You will get a pretty decent range boost for arriving a few minutes later.
I agree with this too. I calculate in advance the expected consumption with a target of 10% but with a generally high speed assumption (80mph is typical, so conservative when in the real world you won't be at your TACC speed all the time) and also using known elevation changes. For the cross country drives I would arrive with about 18% SoC at the next charger on average. Yes sometimes conditions are adverse, so slowing down will keep you out of trouble.
The best way to avoid surprises is to have your energy screen up. On the right side of that screen you will see your expected range if you drive in the same manner as the past 30 miles. This can be compared to the remaining miles to your destination on the nav system. The difference between those numbers should remain stable. If it's narrowing other than temporarily for a hill climb you may need to slow down.
There are some fine points. I'm OK with arriving at a supercharger with 5-10% if I'm immediately plugging in. But if your nav target is a restaurant, sightseeing spot, etc. the target should be at least 20% even if the upcoming supercharger is not much farther. My car announces warnings when turned off below around 15% and also turns off cabin overheat protection. Spending a summer in Florida also taught me that the vampire sitting out in hot weather is not trivial. The car can easily lose 30 rated miles while parked in 8 hours of direct summer sun.
