I checked this with the plane laying on its traveling side on the table saw top. Then using a Woodpeckers tri square that is used only occasionally to check my daily users for accuracy. Auto feelers were used to check the gap.
It might be more accurate to hold the stock of the tri square on the traveling side of the plane to check the sole's variance. There could be some inaccuracy entering your measurement from the table saw top.
jtk
"A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
- Sir Winston Churchill (1874-1965)