I looked up the Trail Boss load for 38 Spl. & .357 in my Lee Reloading Manual.. It shows a max. load of 4.2 for both the 38 & 357 with 158 gr. lead bullet.

804 fps @ 13700 psi for the 38. And 865 fps @ 15867 psi for the .357. ??

I really don't have a dog in the fight. I don't use Trail Boss and I don't really care for, or use, the Lee Reloading Manual..
But it was the only manual I have that had the Trail Boss load in it for the 38 & 357.