I use an RCBS bottom pour electric pot. When I cast, I always leave a small amount in the pot (it helps to keep from overheating the pot leading to short life as opposed to running it dry). The small amount of molten lead, when I start another batch, also speeds up the melt when filling the pot with bullet metal ingots.
1- Pot is cold, has a couple of pounds of bullet metal left in the bottom from when I shut down.
2- I have a large scale sitting on the bench near my pot. I also have a small postal scale sitting next to it.
3- I weigh the cold bullet metal before putting in the pot on the large scale. Record the weight.
4- Fill the pot with the weighed metal
5-Turn the pot on and wait thirty minutes (I also put the mold on my electric hot plate and turn it on to heat the mold while waiting for my lead pot
to come up to heat.
6-My bullet metal is clip on WW's.
7-Return to the pot (all bullet metal is up to heat except what is needed to to bring the pot nearly up to fill limit). I have a two burner hot plate. One
burner is set for the bullet mold, and one burner is set to pre-heat extra bullet metal ingots. The "extra" bullet metal is heated so there is no
condensation with cold ingots to explode the lead pot when adding cold bullet metal to molten metal.
8- All basic metal is added after weighing. It sounds laborious but takes no time and little effort. Then, it is time to add the tin. I know how much "new
metal has been added. I have a pocket calculator and quickly determine how much tin is to be added. My tin is in small muffins. Say I need six
ounces of tin added to give me my 2%. I simply pick up a muffin, weigh it on the small postal scale. Then, I use a pair of water pump pliers to hold
the tin muffin and simply melt off what I need (checking it with the small postal scale). This, again, sounds laborious, it is NOT! It simply takes a
minute and I am done.
9- We all know that using this method it won't be EXACT (as if I were a scientist) but is VERY close. I keep the bullets from each pot separate, so that
I don't accidentally load bullets from different pots that might vary. Using my "system" seems to be overly careful, but I cast match quality bullets
all the time using this method and bullets in the same batch weigh the same and are the same alloy, etc.
Short answer to the original question, just simply melt off the tin ingots to give you the added tin you need to bring up the alloy to 2% tin.
Sorry for the extra details, but hopefully it will be useful information.
FWIW
Dale53