Typically I use the floor to fender lip measurement most of the time.
But, because the body tolerances on these cars aren’t very good even from one side to the other of the same car it can be useful to actually measure the suspension height, which is more important and more accurate.
So it can be worthwhile to actually measure the A-B suspension height like the FSM says. Especially setting the suspension height up for the first time, so you have a better idea of how your car compares to stock and what you might need to address because of that.