If I want to pour exact amounts for my bars, I melt my raw bar into shot/cornflake by slowly pouring it into a large stainless steel stock pot with several gallons of water. I then pull the shot out and weigh out the size I want into beakers. The last time I did this, I had about a kilo of fine silver shot and poured 100g bars. This was mostly to practice technique. I would just pour the premeasured shot into the crucible, melt it, pour it, quench it, weight it, and stamp it. I was usually within a gram or two.
If I had a front opening furnace, I imagine I would just load the weight I want into the bar mold, heat the whole mold up to melt it, and then pull the mold out to cool.
I had also given thought to buying a little Sherline mill and milling the bars to perfection, but I read somewhere that people get really suspicious about milled precious metal bars. I don't know if there is any truth to it. Best advice I got was, "Just get it close enough and then stamp it. It is what it is."
Elemental