DPO questions

Genevieve • 26.married.makeup artist/esthetician. trying for my first little baby
When do you begin staring dpo? Is it counted from the last day of the fertile window, or the most fertile day in the window?!
I want to test but I don't want to do it again too early. 
I tested Friday and got a Bfn. AF is expected to come next Thursday. I'm so anxious. I've had slight symptoms but I don't know if I'm just psyching myself up. My husband and I had very strategic sex trying to better our odds in my fertile window. This is the first month we are ttc but I want it so badly!!!!