a couple things..
I built my -4 and did the usuall calibration scenario of adding fuel a gallon at a time while reading the guage which gives you a reasonable accuracy understanding. You could do the same, but keep in mind, the aircraft has to be in a flight attitude level,ect. The dipstick will be with tail on the ground and give a different reading as you may expect. Also, the fill port is at the outboard point of the tank, and a dipstick will only read down to a point because of dihedral..so, it takes a few movements of the plane and adding fuel from empty to get accurate data.