Jack. To answer your last question first, mathematically, I'm a dumb-dumb, but I think your figures are right. You need just enought tin to make your castings fill out. Any more is a waste.
You use the larger sizing die so that the sides of the treated bullets are not touched.
I guess this goes along with your first question. To be perfectly honest, I don't know. I have never tried it. I just do it the way I do, because that's what the instructions said to do.
There is an article on (www.sixgunner.com) by a Paco Kelly on hunting and stuff with cast lead. Try(www.sixgunner.com/paco/favorite.htm) It's a real interesting read. I guess, according to him, I should throw out my almost 300 pounds of wheel weight metal and use his alloy. He water quenches by dropping directly from the mold into the water. He later lubes and sizes in one operation with no apparent problems. Frankly, water anywhere near molten lead scares hell out of me. I know what happens when even teeny-tiny drops of moisture hit lead from firsthand experience.
I did try his method, but I got erratic reading with an LBT lead tester. The hardness varied by too many points to suit me. With the method I use, there is little variation in hardness between individual bullets.