found a getting pregnant PDF!

Hi guys , so I have been looking into TTC#1, this is my first cycle , AF came April 5th, left April 8th. Always had a regular cycle. Always beteeen 28-35 days MAX ! However I am concerned I may not know exactly when I am ovulating and this month is apparently too late to opk. So I found a PDF file on getting pregnant and they said use the calendar method ! This is what my calendar would look like . Has anyone else used this method ? Any tips or tricks? Thanks in advance ! Xo