I do the following math :

1-First note the %SOC

2-set your car to ideal or rated (like mknox, I have yet to see a difference in my car)

3-Open the energy graph and note the 3 set of values (don't know them in miles but on my car it's 10, 25 and 50km). Note the average wh/km (or mi) and km or mi projected.

Example :

191 Wh/km for 245km projected 186 Wh/km for 251km projected

202 Wh/km for 231km projected

191 Wh/km for 245km = 46795Wh left

186 Wh/km for 251km = 46686Wh left

202 Wh/km for 231km = 46662Wh left

4-Round of the number : around 46.7kWh

In that case, my car was at 59%SOC. So, according to this calculation, 59% (which might be rounded or simply truncated, I can't say) is around 46.7kWh left in the battery.

Now, since the %SOC does not have digits, you have to do this a few times to get a real guesstimate...

My spreadsheet tells me that each 10% SOC is between 7.7 and 7.9kWh.. and total usable kWh in my 85D is between 77 and 79kWh. This seems to match the rating of my car. The 85D's rated Wh/km is around 180Wh/km*. Rated range is 435km. 435km * 180Wh/km = 78300Wh or 78.3kWh.

I think this is a solid estimation. Then, you can start speculating : How much energy there really is in a 85kWh pack? How much is kept in reserve for anti-bricking... we have no clues. We'd have to accuratly measure the total energy available on a __brand new__ model S cell (with a very slow discharge rate) then multiply that by the number of cells

*How do I know this? I did 2 things : pixel count the screen to estimate the height of the "rated" line in the energy graph according to the scale on the Y axis AND drove the car until the RATED and Projected were the same value --> average was then 180Wh/km.