Saying that THCA contains 87% THC by weight would be like saying ethanol contains 70% methanol by weight wouldn’t it. You don’t buy liquor labeled with total methanol content.
Also, if you have to perform a chemical reaction (thermal degradation), I guess in this case to calculate the total. You could add CBD too as “potential total THC” at whatever rate the weight changes when it’s isomerized to THC, to be fair. Am I right?