I'd go for the commercial plastic tube, for several reasons...
*** How do you measure BOTH SIDES with a piece of wood, if the first side you measure holds more than the second side, and has wetted the wood?
*** Surely the wood absorbs a lot of fuel and either smells, or is a fire hazard in the cockpit.
However if you really want to be accurate with the wooden dipper, then just compare it with your mates plastic gizmo....
btw.. I think each model 172 is different, especially the SP version.