for me, especially because i have a mood disorder, my periods aren't very regular. but i still find it helpful to have a ball park figure, so i count by 28 days also.

so if i get my period on a wednesday, then i count 28 days from when it first comes (i.e., not when it ends). usually my periods will fall a few days earlier/later than the predicted date or sometimes they will not come at all, but i know my sisters get it like clock work.

like raph said though, your body might take a while after coming off the pill also to adjust.