28 days from the first day of your menstrual cycle. 

I am not sure what you are hoping to accomplish.  I kept tract of my menstrual cycle for years, and I would count from the first day of one period to the first day of the next.  It varied anywhere from 24 to 32 days. 
There really isn't any reason for you to count forward 28 days unless you want a general idea of when your next period might begin.  Tract your cycle as well as counting 28 days ahead.