That is why we use the OPK. If your cycle is not regular you will still have a hormone surge about 36 hours before ovulation that will be detected by an OPK test and then you'll know to DTD then. Or, if you prefer, you can just try one day before you expect O if you're regular. whichever works for you.
Please don't overthink being 35. So many of us have been thrown into a panic by these dire reports on waning fertility over 35 that we have been afraid to sway. But I'll tell you my story, which is that I thought I had to do all this stuff to get pregnant at 37 and got pg in 2 months with my 3rd boy, then I thought I surely had to do all this stuff to get pregnant at 39 and got pg in 1 month with my 4th boy, and then at 41 I did nothing to help my fertility in any way and got pg in 6 months (which is still normal time range) with a girl. It is not the way that the media makes it out to be and you can read a full scientific debunking here
http://genderdreaming.com/forum/late...d-article.html