THE CORRECT way to get the center of the tire is to scribe it.
with the wheel off the ground mount securely something sharp like an ice pick to something solid. move your new tool to the approximate center of the tire and rotate the tire to scribe a line all the way around. that is the line you measure to. and yes the point of your measurement has to be at the same height front to back.
a member of our club does almost the same thing. He jacks up the front end, installs masking tape around the center tread of the tire, uses a pen or sharpie to mark center line on tape, and then proceeds to measure toe-in. the tape and marker guarantee a straight line vs the tread of the tire.