Here is my thought process.
I ordered a 70D with a physical 75 kwh battery in it. It is obviously software limited to 70 kwh. The advantage to keeping it at 70 is I can charge it to 100% each time and not effect the long term effects on battery life (confirmed by Tesla Service).
So, currently I have charging set at 100% which translates to 240 miles. But 240 miles is not actually 100% of the physical 75 kwh battery. So, I won't hurt the battery when I charge at 100% each time. If I were to upgrade to remove the software limit then I would get 257 miles or so. But the recommendation is to run the charge at 80%. They even have that nice little suggested line in the graphics to show where to do that. But 80% of 257 miles turns out to be 205.6 miles. I would have to manually charge the battery to 90% or so to protect the 75 kwh unlocked battery. 90% of 257 miles is 231 miles. I still lose 9 miles. I would have to then manually charge to 93% to get back to 240 miles for a charge. Personally, I would rather just slide the control to 100% and get 240 miles rather than mess with the details of worrying if I got 90 or 93% charge just to get what I automatically got when my battery was at 70 kwh at 100%.
Plus I get to keep $3250 upgrade charge in my pocket. WIN-WIN
I'm a little puzzled by your logic. The 70kWh and the 75kWh battery are the same hardware except the 70kWh is software limited. The only thing that really matters is to what percentage of the total capacity of the 75kWh battery you are charging. Using your numbers, if the total capacity in miles of the unlocked 75kWh battery is 257, which is about right, and you are charging to 240 miles, you are actually charging to 93.38% of the total 75kWh unlocked capacity, which Tesla does not recommend on a daily basis. You're daily max should be 90% of the 75kWh 257 mile total capacity or 231, which is be 96% of the 70kWh capacity, not 100%.
I do agree that there is some benefit to being able to safely charge to 96% rather than 90% if you are facing daily commutes that take you to the edge of your range. However, if that is the case, you might need the 75kWh battery for the extra 17 mile range.
I never charge my S70 more than 80% unless I know I am going somewhere that requires more range and vary the percentage charge below 80% periodically.