DonnyFL, and others I reckon, suggest buying the the 'next caliber up' of whatever you are shooting. The Taipan Veteran uses the same opening, in the baffle pellet path, in .177, .22 and .25. That opening is .285"
You can start skinny and see if you get clipping and how the accuracy is. If it clips or you get fliers, of course testing with a known performing pellet, then make the orifice larger.
The likelihood of clipping increases the further the pellet is from the muzzle. They PERHAPS do not all need to be the same size but they do need to be dead-on centered.
Poor accuracy or fliers does not necessarily mean it is clipping. Bad shroud guts can create turbulence. You have to inspect the guts carefully.