How much

[rfd]

i *think* the formula is 1 (representing the percentage of tin) divided by the target pecentage of the lead.

so that a 1:16 tin/lead alloy would be 1 / 16 = .0625 and a 1:20 allloy would be 1 / 20 = .05 and then .0125 of tin added to the 1:20 would yield a 1:16 alloy. phwew!

here's the numbers i came up with if starting off with 5# of pure lead ...