10% on the top part (Treble clef, stems up)
-Barbershop tenor. A few of your Sopranos. Better to have too few than too many.
20% on the third part down (Bass clef, stems up)
-Barbershop baritone. Your super smart kids from Alto and Tenor. Remember, this is a difficult voice leading part with tri-tone jumps.
30% on the second part down (Treble clef, stems down)
-Barbershop lead/melody. All the other sopranos, altos and a few tenors.
40% on the bottom part (Bass clef, stems down)
-Barbershop bass. All of your basses and most of your tenors.