How accurate

My husband and I have been trying for 3 cycles now. So far we have come up negative. This month we had sex every other day durring my window. I am 1 day before period and this am decided I would test. I got one of those 99 cent Walmart pregnancy test. The one with the dropper. Well it came back negative. 😔. But I'm wondering how sensitive it is to pick up the hormones? It said 99.9% accurate on the box but it doesn't say on day of missed period.