Online ovulation calculators?

My BF and I had an accident the other day.. According to an online ovulation calendar, it happened 4DPO. My question is.. How accurate is that? For someone who has regular 28 day periods would that mean the ovulation day would be pretty accurate too?

count down to pregnancy.com

If you are past your ovulation it is likely nothing to worry about but if your body decides to be all fun with you and throw your ovulation off you could be screwed. I would say take a test about 10 days after the accident.

I've always had 28 day cycles and been right on with my ovulation (except for this one time, ha.)