When to take a HPT?

So, my LMP was on 9/8, but I'm not sure when I ovulated this month as I haven't been charting, and my OPKs (which I started using on CD 13)weren't showing any "true" positives. However, DH & I were very good about BD'ing this month! I had some mild cramping Tuesday & Wednesday (none today so far) that I am *hoping* may have been implantation. My question is, when should I test? I am thinking my period is due in 4-8 days or so, but because my cycles have been slightly different lengths each month since I miscarried in March, I'm not positive. When do you think would be a good time to test & expect an accurate result? TIA for any advice!