If I'm a day later than my longest cycle...then I consider myself 1 day late.
I've only been there once where my cycle came on CD34 or 35 (I forget) when I average at CD33 for AF. If you're TTC, I'd say if you haven't gotten your AF the day after expected period, then test the day after that. Although if you're preggo, you should be able to test the first day of your missed period.