Thanks for the query to H.C.M. Forum.
Normally 5 /28- 30 days i.e. bleeding for 5 days and period of 28 - 30 days.
Calculate 28 days from 1st day of menstruation
( 1st day of bleeding) .
Now onward calculate irrespective of months ( count 28 days and next period ).
Hope you have understand this calculation .
if not I will try again.