Yes, you've got the right idea. You can approximate the usable battery capacity as you have done.
The consensus around here is that usable battery capacity of the FFE is anywhere from 18.5 kWh to 19.5 kWh. So, your result of 18.9 is certainly in that ballpark.
You'll note that the window sticker for your FFE said that it can go 76 miles on a full charge and does so getting 320 Wh/mi. But, that 320 figure, quite usefully, is the full "wall to wheels" energy required, and accounts for any energy lost during charging, which is typically about 20%. (Yes, EV charging, on average, is only about 80% efficient -- in simple terms, some energy lost as heat.) Of course, this wall to wheels amount is the true amount of energy, and is what you'll have to pay your electric utility per mile, to drive your FFE around. It makes sense to put that number on the window sticker.
The "battery to wheels" energy usage, on the other hand -- what the car displays to you in trip meter and other energy displays -- does not include the energy lost during charging. The car's accounting only reflects the energy stored in the battery (which, as we know, is only 80% of what originally came out of the wall). But, we can use the information from the sticker to deduce that, if you were to drive 76 miles on a full charge, the car should display an average Wh/mi of about 80% of 320 Wh/mi -- or 256 Wh/mi. Indeed, this 256 figure has been corroborated by people who routinely achieve mid-70s ranges driving around in their FFE -- sure enough, their average Wh/mi settles somewhere within 250-260.
Anyway, at this point (quite similar to what you have done with your trip meter numbers) we can estimate the usable battery capacity:
256 Wh/mi * 76 miles = ~19.5 kWh
Now, to be sure, 19.5 kWh is the upper range of what anybody has seen using the "trip meter accounting" technique, but perhaps the FFE is not even 80% efficient when charging, it may be worse, or perhaps the trip meter isn't completely accurate.
