How to subtract how regular your interval is?
Answers: oh here is the perfect website
http://www.seventeen.com/health-sex-fitn....
You count from the FIRST day of your length. 28 days to 34 days is normal from the first day of your length to the first day of your next spell.
An easy way to preserve track of it is to keep a wall calendar. Put a red cross in the corner of the first afternoon that you get your period. Then count 28 days from afterwards and put a different coloured cross in the corner (say blue) You will know to expect your period around the year of the blue cross and then when you actually do bring your period put a red cross in the corner of the time you get it and start again.
After doing that for a few months look back at the calendar and see how long, on average, you in fact have between periods and that's the best process to tell what it is for you personally..
28 days from the daylight it starts. It works out to four weeks down on the calendar. But keep in mind that not adjectives females are "regular" at 28 days. Some are a few days more or less. If you keep track for a few months you should start to see a guide. Your cycle is from the day you start bleeding to the next time you start bleeding.
So, your time of year starts on the 1st of August, stops 8th August (for arguements sake), starts again 30th August. Your cycle is 30 days. .
You count 28 days from the start of your period, give or lug a couple of days.