Advertised diameter is not necessarily what the as-cast bullet diameter will be. Depends on the actual dimensions of your mould and the alloy you use. I would start with a bullet sized to .225", if such a bullet in a loaded cartridge will chamber without difficulty. If not, try a .224". For a cast bullet, you generally should use a bullet no less than a thousandth larger than bore diameter. Check chamber throat fit as well. If you can push a bullet through with a bit of effort, it should be okay.

Lyman books for years often suggest sizing to groove diameter and I don't know why other than a groove diameter bullet will almost always fit easily, but it usually won't be the most accurate bullet and it will likely cause leading.