I have a new rifle(to me) in my safe, got it as part of payment for a job I did.

It's a Mkontho in-line rifle based on the Pauley design(plunger), they are build in South Africa by a local gunsmith and proofed up to 150gr loads.
The rifle has a 1:24 twist, 21 1/2 inch barrel(measured from crown till touching breechplug), .451caliber and ignited with a standard no11 percussion cap. No open sights, fitted a old Bushnell 3-9x40 scope.

I have slugged the barrel, 0,457" and 0.452" measured. I have the Lee .457"mold that cast them at 405gr depending on the lead. I also have a .452" Lee re-sizer. I want accuracy and not so much speed as the rifle are supposedly very accurate at longer distances. From what I've read so far the ideal speed should be around 1250-1350fps with the Gibbs target rifles, I guess it should be the same with my rifle.

Edit: Let me rather re-phase my question. How do I determine the ideal size with the measurements above to be able to shoot the rifle accurately without having to think about paper-patching? Lee make a .452, .454 and a .457 re-sizer but my bullets already are .457".