The 60% reduction only applies to certain powders. Hodgdon only advises it for H4895.
Hodgdon's formula for working up loads with Trail Boss in any cartridge is on their website. It's easy to use, but I dunno what velocity you might get. Trail Boss usually results in pretty mild loads, thought it might work with a .225. You don't need a filler.
When I had a No. 1 .450/.400 I came up with a few milder loads. One was the .300-grain Hornady softpoint for the .405 and 80.0 grains of H4350, which got around 2300 fps. I also used a cast Lyman bullet that with the alloy I used weighed 408 grains. It shot well both with 100.0 grains of Swiss 1-1/2F black powder and with 32.0 grains of Accurate 5744. Both got around 1400 fps, and the 5744 load would probably work well with a jacketed 400-grain bullet, though velocity would be a little lower, due to not sealing the bore as well as a cast bullet.
There's also a formula for working up loads with 5744:
1) Fill a case with 5744 to about where the base of a seated bullet would be.
2) Weigh the powder.
3) Use 40% of that powder charge as a starting load.
Id bet 5744 would work with a 225 or 300-grain bullet and get the sort of velocity you're looking for.
