W&B-Where do you Measure People Positions

I'm helping a chapter member do the initial weight measurements on his
plane. I think we have everything except what point should be used for
the seat positions. I didn't see it addressed in the weight and balance
section of AC 43.13.

What point along the seat is considered the center of a sitting person?
I know an inch or two either way won't make a bunch of difference, but
we might as well start with the best number achievable.

Thanks