this is all I have been able to find on the web regarding .455 and trailboss, the stuff below I have taken from different forums, use with caution
In a nutshell, 4.4 grains of Trail Boss (Lee disk 1.02cc) gives 640 fps from a 4" bbl, with a max spread of 20 fps. The powder fills the case about 3/4 full and is slightly compressed by the head. I estimate Trail Boss to be roughly the same burn speed as Unique. Burns very clean. Meters in a Lee disk powder measure very consistently. In short, I think it's a great powder for old BP guns. I am very pleased with it.
The all important detail :
Head - 265gr hollow base .455" cast from NEI mould #300 with 20:1 lead/tin alloy. Nominal weight of 268 gr. Sized to .455" in a Lyman 450.
Brass - Winchester 45 colt brass, cut down to Webley 455 Mk 1 spec of 0.86".
Winchester Large Pistol primer.
LOA of round when case crimped into crimp groove is 1.363".
Air temprature was 96 degrees. Dunno about humidity, etc. Altitude is Austin Texas !
It's IMPORTANT to note that these are Mk 1 spec rounds. The brass is longer then the Mk 2 brass. Mk 2 brass is about 3/4" of an inch. I haven't loaded Mk 2 cases, so can't offer any information on them. If you have a .455 that's been shaved for 45ACP, then this load data is valid for Auto Rim brass as well.
With 230 .452 RN and RCBS 265 HB using 5 grs Unique, my Colt NS prints 6-8" high at 15 yds off hand.
In desperation I tried 4.5 grs Trail Boss with the 230 RN - Bingo! It printed right on POA. For pistols older than the MK IV and MK VI Webleys, I'd cut that back to 4.0 grs. The loads given in the first post were not intended for the .455 Webley MKII cartridge case, which is afaik the only current commercially available .455 webley headstamped brass.