No science behind 20 rep squats, its just damn hard work.
Most dont quite grasp how your meant to perform on them though.
If you start, and dont get to 150% of your bodyweight, you really havent worked hard enough for long enough to understand and reap the benefits.
I see guys doing 20 reppers with bodyweight, and saying there tough. They may be now, but your not finished yet. Far from it.
Its no point starting them and ending up doing 80kg x 20, see it through.
Watching lifters do 20 reppers tells me all I need to know about them.
I think double bodyweight for 20 reps is a good point to finish for most, some freaks can push it much harder, but the average lifter will do well to do double bw x 20