If you are having fill out problems I would recommend adding tin. What I did was add some solder, 95% tin 5% antimony. Adding more than 2% is a waste in my opinion. I have a roll of 1/8" round 95/5 solder. I add 24" to a ten pound pot.
CrustyFN - Thanks for the Tin / Antinomy tip!!!!! I buy my lead from a local metal recycling place in northern dallas and have been casting bullets with just the plain old lead. I've always known that it was a good practice to add tin and antinomy, but couldn't really find any to add (the metal recycling place doesn't have this - surprising). I've got probably 10-12 rolls of solder laying around, so as soon as I finish typing this reply, I'm going to do some more casting.
IllinoisCoyoteHunter - great post my friend. I have a nearly identical process that I follow, so it was nice to have my mehtods confirmed by an expert.