I had three heads on when I thought about the answer to this.
Head 1 - Go for the higher one, you can have more...more...more!!!
Head 2 - Go for the lower one, then you can't have too much.
Head 3 - go for an average, none of its an exact science, so just average it!
No help at all am I. Just think yourself lucky, you don't have to deal with my three heads on a daily basis.