Thanks for the well thought out answer. I’ve been playing around with ABRP and what you said seems to fall in line pretty much exactly with ABRP. I set the speed to a constant 75 mph, 85 degrees, and default 5% degradation. It’s saying you can make it about 255 miles from 100% to 0%. If I keep it at 0% degradation then you can get 270 miles. I’ve grown very fond of ABRP lately and the amount of customization you can do to get a result that works for you.Those aero wheels and tires help considerably, so I'd expect more like 270-280Wh/mi for the LR for that scenario. (I don't have one, so I am definitely guessing.) My brother has a Stealth though, and this seems in the ballpark from the numbers I've seen him get for ideal, high-speed Seattle to Portland runs (250Wh/mi) - a little slower than 75mph on average though. Obviously other cars on the road here is a factor - I would not expect this low on a deserted highway.
For reference, I recently did a 2600-mile trip in my Performance from SD to Seattle & back (but was not on interstate most of the way (395 and east of Sierras/Cascades), so was usually at more like 65-70mph), and I averaged 270Wh/mi with a fully loaded vehicle. I had CrossClimate 2 installed which are not as good as the MXM4s for efficiency, and my wheels are not aerodynamic at all (Konig Dekagrams).
Anyway assuming 270Wh/mi, for the new 2021 "82kWh" LR you have 76.9kWh to work with (not including buffer) and 1% heat loss somewhere, so it ends up about (76.9kWh*0.99)/270Wh/mi = 282 miles.
But that's to 0% (but still, doesn't include the buffer), so if you go for 5%, which is not particularly stressful, you end up at ~268 miles (270Wh/mi, 100%->5%).
On prior generations of the car, this worked out to be 260 miles. With capacity loss, most of those cars can now do about 235-240 miles. (I can do about 235 miles at that consumption to 5%.)