Tell ya what normally happens with me... I put the saddle on, go to the off side, pull it up, or pull it down, and if it's not at least halfway up on the (sorry, I'm on my first cup of coffee - mind's not working yet)... left side, then I measure that one, and figure out what length I need.
I would imagine you sould measure from point A on the right side of the horse to point B, and then take into account if the girth is a bit stretchy.