Measuring upwards curve?

New here, so this may have been asked before, but is the method for measuring an upwards curve the same for any other? I guess my thought is that there’s simply going to be more length on the underside (outside) of the curve than there will on the inside of the curve (a little over 7’) I’ve seen people say just straighten it out (not possible), and I’ve seen some people say measure both the top and the bottom and average them? But then, I worry that I might place the tape too far back and give myself more length than I actually have.

Since the outside of the curve is obviously going to be longer, is that still considering “usable” length since it would still be utilized in sex the same way the inside would? People with a downwards curve still measure from the top, and they’re measuring the outside of their curve, so shouldn’t a guy with an upwards curve do the same?

Is there an actual method to measuring for an upwards curve that’s accurate/doesn’t have room for “cheating”?

Sorry if this is repetitive