When do you use preseed?

So a little back story: We had an easy, healthy conception, pregnancy and delivery with our daughter who is now 7. I got my BC removed in August and we got pregnant after one cycle. After a MC in November we are TTC again. It seems like I got pregnant so easily before and now it’s like rocket science. Before my miscarriage glow was extremely accurate with my ovulation dates and period. Since my MC things have been a little off. My period started and entire week early this cycle. So since I can’t rely on glow to accurately predict my ovulation I broke down and ordered a 30 day opk. I’m super excited and really curious to find out how far off my calendar is. Anyway while I was a the store grabbing some laundry soap I decided to wonder over to the feminine section and picked up the infamous Preseed. I figured why not. What I wanna know is do you use it every time you have sex or just during fertile window?