You've basically got the numbers correct: your battery appears to have a capacity of 78.75 kWh .Using the method if i didnt misunderstand i got 78750kw? Is it correct?
Should be accurate to within a percent or two..
For comparison my 2016 P90D is currently around 75.2 kWh..
The older 90 packs were notorious for actually providing well less than 90 kWh, even when new; they started with more like 83.