I did that before but I was afraid I could loose some fuel if filled too much to the top. I once had some fuel flowing out of the wheelwell. Filled to the top and probably took some roundabounts a bit quickly right after.
Small amount is relative, official tank size is 43 L. When I put in 35 L, I consider it "full tank". I agree, though, that I'll have half of the accuracy of someone who fills 70 L. The AX has a big hole, about twice the size of the fuel tube. And I orientate the tube so it flows against the sidewall and not directly straight into the hole, so when it clicks I believe it's near top. By filling up with this method, my tank gauge always starts to move after about 220 km for consumptions of 2,8-3,0 l/100 km. At similar conditions I always get close consumption figures, never like 2,6/3,1 but like 2,8/2,9. I think it's pretty consistent and reproductible. The fillup after that is 2,82
, so it was (near) full.
EDIT : my signature shows now 3.0, yeah ! 3.04 exactly, hope to be at 2.99 in one year.