My wife uses a spreadsheet and connected it to her calendar. Seems pretty accurate.
It is a modified version of this:
http://www.alizaaufrichtig.com/period-tracker
UK too: https://data.gov.uk/
My wife uses a spreadsheet and connected it to her calendar. Seems pretty accurate.
It is a modified version of this:
http://www.alizaaufrichtig.com/period-tracker