BC .243 +/-
What tin does is not so much a matter of changing melt temp, but of lowering surface tension in molten lead. It doesn't take a lot of tin to make things work, typically 2% by weight is enough.
I think the most common cause of rounded bases is a cold sprue plate. When heating the mould, put the sprue plate down.
A suggestion to you Luddite types....get a farkin' thermometer. Life is too short to spend it fixin' up the mess your guesses created.
I asked Santa to bring me one this year. I've been trying to get along by using a infrared thermometer, but readings are just not consistent enough.
