The day I dread the most all year is the day we pick our replacement heifers. Ideally, I would like to keep them all til they raise one calf. We just don't have the pasture to do that. So, we use all the tools we can, but selecting replacement heifers at weaning time is basically a crap shoot.
We have culled our cows really heavy, so the heifers should ALL be good ones. Of course there are always a few that we don't like as well.
Interesting, they had a 4H judging contest in town on Sunday and we were asked to take some yearling heifers in. They wanted 8 for a 'Pick and Cull' contest. (Pick 4 and Cull 4). We had a hard time getting 4 and 4, or at least that was our opinion. We selected a heifer with a high tail head as a 'cull'. The judge picked that heifer as the fanciest heifer in the pen!! Goes to show what one person likes, another does not.
So, it is hard to say whether or not you keep the top 20%. Everyone sees cattle different.
What we do is take out so many, let the buyer take out so many and then we take out another draft, then they do. That seems to work really good when we sell replacement heifers, being fair to all.