Hey K
You know your cycle is usually 28 days however what you haven't told us is the length of your luteal phase (from O to AF). This is the only constant in your cycle... your follicular phase (from AF to O) can vary from cycle to cycle, but your LP usually stays the same (however this isn't true if you actually conceive and lose bub soon after conception (((hugs))) ).

For example, my cycle can vary from 26 to 30 days, this means my O day falls anywhere from CD 14-18 and my LP will always be 12 days, with my period arriving 13 DPO.

To get an accurate test result you would test at the end of your LP (for me 13 DPO).

Does that help or hinder?