Problem with eyeballing to the center of the riser is that very very few bows have the string in the center of the riser. It's always offset to the left (right handed bow) about 1/8". That's for the dynamic center. Of course then you have single cam bows that the string angles from left of center at the bottom cam to the center at the idler wheel.

Walkback method does work very well in almost all applications, though.