That is a Great Tip Grumpy !thickness is of no consequence, its just a common circular disk that fits in the blocks lifter bore,
rides over the lifter and won,t compress as would a hydraulic lifter seat,most chrysler lifters are about .904 dia. pennys are about .75, but larger than the inner lift seat diam.
your not concerned with anything but measuring the lifter movement in relation to crank/cam shaft rotation