I've flown many variants on this aeroplane, and it's always been done on one (or both) of two bases:-
(1) Dip it
(2) Minimum known value based upon either tabs or top (no other assumptions made).
It's perfectly possible and sensible to use a dipstick, because of the dihedral it needs to be calibrated, but that's not difficult. Most club operators have a calibrated dipstick for each aeroplane, if they haven't I'd argue it's inexcuseable.
Finally, anybody telling you that the electronic gauges on Pipers are believable and accurate is talking utter b****x.
G