I think 20% must be an average and not necessarily true for any individual.
Dd was conceived on the first try, 3+ years ago, (without even knowing when I ovulated), and #2 was conceived several months ago, without even trying. @@ In fact, dh and I had sex 5 full days before I ovulated, and not again until well afterwards, and I got pregnant from that! I did FAM, and I had clear and multiple indications of when ovulation and implantation occured, so I'm sure of my dates here. I guess I'm one of those very fertile women. And dh has some "super dudes" as he puts it. I'd always read that the absolute longest sperm can live is 5 days but that it is rare.
So, if it was me, I'd take the 20-25% figure with a grain of salt, and I'd temper it with a knowledge of my personal and family reproductive history.
Catherine