It's really easy to calculate. Just multiply recent efficiency Wh/mi by projected range at 100% condition.
Go to the trip graph where it shows your recent efficiency. Take the Wh/mile number along the left side, multiply that by the remaining range along the right side and then divide by your current battery % (change your display to percentage).
Example: 222Wh/mile * 150 miles remaining (from graph, not the battery icon) = 33.3kWh remaining. Now divide that by your current battery percentage remaining. Example, at 50% divide by 0.5 and you find your full capacity is 66.6kWh.
This method has been proven to work.