I shoot cast .38 spls that are 95% Lead, 2% Antimony, and 3% Tin. I cast my .45acps out of the same recipe. I get no leading, even pushing my .38's with 4.9 gr of W-231, a Lyman max load.
I was getting leading shooting pure lead cast bullets, even at slow velocities. (I had to try, didn't I?)
My point is this: Save your solder. It's expensive. Mix everything else together and cast some bullets. Say, 100. Measure them carefully. If they're too small, melt them down, add some of your solder, and cast them again. If not, shoot them. If they don't lead your bore, cast the rest of your alloy.
Trial and error is a time proven way to succeed. Leading is not that difficult to clean from a bore. If they lead the bore, try again adding some of your solder.
In my opinion, and experience, a good soft lube will go a long way when preventing leading. A hard lube won't work as well. Lee Alox works great.
Oversize bullets lead less than undersized bullets, in general.
This is just my experience. I am no expert. I just like to shoot the least expensive bullets I can, and pure lead is free for me.