I think your math is slightly off:
21/2 = 10.5
5 x 10.5 = 52.5
But 121/2 = 60.5, which is 8 lbs too much...
Jk, I'm just giving you shit (not manure). Seriously though, how does this work? I usually mix my dry top dressings with worm castings just cause it's easier to spread out a larger volume of material, does it work the same way using castings instead of manure?