Some umpires use a down indicator or rubber bands on one of their hands to indicate where the ball was laterally so they can return to the right spot on incomplete passes.
In marking off fouls, I always recommend using the lines on the field and not your steps. When I first white-hatted many years ago, my U marked off two consecutive encroachments on the Defense (from 1st and 10) and he ended up short of the line-to-gain!!!
Whenever I umpire, I analyze where exactly between any two lines the ball is. This is true, not only on penalties but also on returning to the previous spot on incomplete passes.