Replying to Measuring True Center

Picked up some 2020 bdogs this summer second hand still with the base sticker on. Getting ready to take it to the shop for a mount, turns out its missing a 'recommeded' mark on one of the skis. Measured with a straight tape tip to tail and a carpenter square, and the one ski that does have a mark is like 2mm behind my measured mark.

Whatever not a big deal, just curious is a straight tape pull is the best way to find center, or measuring with a flexible tape on the ski, or maybe a tape measure on the bottom?

Whats the most accurate? Maybe I'll take this to TGR lol