For the last few years I have smelted (yeah I know its not technically right) my range lead, fluxed, reduced and poured into ingots. Then I weigh the ingots and add my tin accordingly. Doing it this way might be a more accurate way to get my tin percentage but it is also a two step process. Next batch I'm gonna weigh the dirty range lead, (about 140#'s) and start smelting. I have a nice metal bucket that i'm gonna put all the skimmed sand,dirt dross in. Weigh the contents of the bucket and subtract from the original weight that went into the pot and add tin accordingly. This might be slightly less accurate but why waste the energy to get 140 #'s of lead back up to temp when it's already there?
Scott