Well I would definitely stick to .454 at least for the sizing, .455 or .456 is actually better but it depends on the revolver. That amount of trail boss is about right given the known speed of the powder and pressure. Basically the .455 Webley is very close to the .45 Schofield in ballistics and pressure and for 200 gn in the .45 Schofield the IMR website has 4.0 to 5.0 max. Personally I haven't found a cartridge that liked Trail Boss, tried it in .38 spl, .357 mag, 44 Mag, 45 Colt and 30-30. It wouldn't even come close to Unique/2400 but you must have found something that it will shoot in. I would start at 3.0 gn and work up slowly and watch for pressure signs.