The factory seat placement guide is designed to help you place the seat where the weight will be optimal. Starting with their measurements and tweaking from there is your best bet.
It’s a good idea to do as you are and get the driver suited up with helmet and everything to get the most accurate readings too, as you’re planning on doing.
We usually throw a little 5 pound weight on top of the fuel tank to simulate fuel. It shouldn’t really change your front/rear % because it’s so central on the kart.
Team Driver - Innovative Performance/Tony Kart // Owner - Oktane Visual Custom Helmet Paint and Graphic Design