IO540
... hear what you say. Which is why I said "Carefully input your waypoint map references"
I can honestly say that in using my approach described, ie checking the resultant distances against that worked out on the Plog, has always enabled me to identify errors with either my GPS waypoint OR my Plog! The approach is a Gross Error Check in itself.
GrassStrip admits to some uncertainty about his nav skills, and I get the impression that he/she is primarily looking for GPS as insurance, rather than a tool which has equal footing with VFR and instrument methods... hence my suggestion.
Cheers