RE: Help figuring out number of sets with this problem
20% of the weight, that is too light for a warm up set!
Having said that some of my heavier lifts i.e. squats and deads i do three warm ups then 4x5.
If your routine is set out right you will not have to any warm ups other than the first exercise, because that first exercise encompasses all the muscles used that day.
You shouldnt need to re work anything just do your warm up sets for the first exercise and then get stuck in!