I use a sophisticated tool to determine wheel weight material--I scratch them with a nail. This may miss a few, but I had only one zinc weight float to the top while smelting lead. In a subsequent re-culling of the zinc weights, I found two or three lead weights I had missed.
Having weighed two sample ingots, I found they were 5+ pounds rather than 4 pounds as I'd estimated, so my net production was actually 75 pounds.