DPO and testing with pregnancy test

Anja

I would like to know how to calculate DPO. I have done a ovulation test the 21st of March. The line was there but not dark as the control line. I kept testing and it went lighter. I think I must have ovulater earlier like around the 19th. My husband and i did have intercourse the 19th and 21st. How many DPO am i?