You need to perform
weight and balance calculations just like you'd need to do if you were building an airplane. On fixed wing, the datum line is typically the firewall. The the weight is measured, and the moment arm (the distance from the datum line) is calculated and the effect of the weight based on its' location can be known. This is why modifications to certified aircraft are illegal without engineering calculations, and a full new
weight and balance being completed. When I was an aircraft mechanic, years ago now, we had to redo the
weight and balance on every helicopter during it's major maintenance cycle.
Look at this:
http://home.new.rr.com/trumpetb/alph/
Tim Hancock does this way more often than I do. He'd be the one I asked for suggestions.
However, I believe on your craft, the datum line would be the nose.
Maybe.