Help tracking fertility window

Hi ladies! I was wondering if someone with more experience would be able to help track that window this month for me? My cycles don’t range the same everything month. It normally goes from 24-27 days. I’ve had one 28 day cycle in the past year. According to my GYNO that’s still classified as normal? as long as I’m not having 2 periods in one month? I’m not saying I question his word I was just always under the impression that it was suppose to be the same cycle length every month. I’m not sure if the cycle difference changes the ability to track the fertility window. For the last two months they have been 27 days with the last one being on June 25, 2018. My husband and I have been at this for almost 6 years and this is the first time I decided that we would try to track the window but with the cycle difference changing every month or every other month I don’t know if it’s possible to do so