.452" is a good diameter for these things, I agree. The Webley Mark VI notoriously had the cylinder throats bore tighter than the actual bore of the barrel. I believe that it could fire the .450" Webley cartridge as well as the intended .455" Webley cartridge and yet also fire the .476" cartridge. So sizing to barrel diameter is pointless as the bullet is in any case swaged down as it passes through the cylinder.
When I designed both the .455 Mark II (265 grain) and .455 Naval Target Bullet (220 grain) for RCBS back in the 1990s and then had one hundred of each produced and imported these also were set to cast a bullet that should nominally be swaged to about .452" when sized and lubricated. These Naval Bullets being done from copies of the original drawings that the Forensic Science folk at Huntingdon supplied in return for help about something else .455 related. Yes. .452" is a good choice.
The Mark II was done first and then some years later the Naval Bullet. That was originally going to be actually the .455 Mk IV "Manstopper" but the edges would feather on the mould as it was used. So the domed Naval Bullet was the one that eventually we went with as the edges were less prone to feathering being a shallower radius similar to but not alike to a dome of a satellite dish.
