What's your groove diameter? I'm betting that .459" would be just right.
When I size 20:1 .459" slugs to .452" for my 45/90 I roll them on a foam case lube pad, base-first size them .454", then nose-first to .452" through a Lee sizer. If using a harder alloy like wheel weights, it helps to go nose-first through the .457" Lee, then throught he .454" Lyman, then through the 452" Lee. Sounds like a lot of trouble, but it keeps the boolits concentric, eliminates flashing tails on the base, and it takes only a few minutes to size 100 three times. That's a lot of patching and shooting.
Gear