You can go into the service menu and see a little more about the battery health. If you go to Software, then long press the word "Model" then in the popup type in "service"
From there you can go into the HV battery and see a little more. But you are right. To really do anything, you need to do a true battery health test, which includes leaving the vehicle for up to 24 hours plugged into a L2 charger. The vehicle drains the battery and cycles a full charge to report the true health.
The approximate health, you can do what you said. Using math if it says 80% and 240 miles, you can convert the ratio to 100% = X miles.
240 miles / 80% = x miles / 100%.
(240 * 100)/80 = 300 miles
Basically, if the precent is 80% with a mile range of 240, if it was charged to 100% it would have a range of 300 miles. The problem with this, is it accounts for driving style. The true test in the service menu tells you the actual battery health.
Service Menu instructions:
Correction Code 00050100
service.tesla.com