How to subtract when you're coming on your spell?

Right me and my best friend NEVER get this right, so someone please clear it up for me.
Do you count 21 days from the daylight of when you last come OFF your period. So you count 21 days and specifically the date you should come on?
If wrong please tell me how! I know it vary, but ROUGLY is that what you do? Thank you http://www.seventeen.com/health-sex-fitn...
^go here
It will ask you when u last get your period, how heaps days it lasts, and average cycle.
It will give you the month and make clear to you when your ovulating and when your schelduled to get it!!
Good Luck!

EDIT: okay sorry I in recent times thought it might be helpful to you! You count rotten 21 days from the LAST day you get it. for example if the last daytime you ended be december 8th you would count 21 days from that and that should roughly give you the time your interval will come!
Ask your men they will tell you lol