I sent an email to Hodgdon powders about using Trail Boss, below is there response.
Bob

Trail Boss offers superb versatility in rifle cartridges producing reduced loads using lead or jacketed bullets. Reduced loads make firing such cartridges as the 300 Winchester Magnum or even the 458 Winchester Magnum pure fun!
If you don't see Trail Boss data for your favorite cartridge we have a formula for developing loads for all cartridges and it's simple to follow. This formula may be used in both rifle and pistol applications:

1. Find where the base of the bullet to be loaded is located in the case and make a mark on the outside of the case at this location. Then fill the case to that mark with Trail Boss, pour into the scale pan and weigh. This is your maximum load. Pressures will be below the maximum allowed for this cartridge and perfectly safe to use!

2.Take 70% of this powder charge weight (multiply the maximum load from step 1 by .7), and that is your starting load.

3.Start with this beginning load and work up to your maximum charge, all the while searching for the most accurate reduced load.

Dave Campbell
Customer Service Representative
Hodgdon Powder Company
6430 Vista Drive
Shawnee, KS 66218
913-362-9455 Ext. 117
dcampbell@hodgdon.com