Seldom work Umpire myself, but do a lot of rookie training so I cover this issue with them. You are marking 5 positions - left hash, left post, center field, right post, right hash.
All the Ump's I ever work with used one of two methods,
Thumb and four fingers - Thumb for hash, index for post, middle finger for center field, ring for post, pinkie for hash.
Just four fingers - index for hash, middle for post, loop it over BOTH middle and ring for center field, ring for post, pinkie for hash.