Thank you everyone. What I did, is measure the 158 gr swc bullet and subtracted the length from a new .357 case. I then machined the case down to this measurement. I filled the shortened case to the max with Trail Boss and measured the charge.
I just finished shooting a 4.5 grain load of Trail Boss in .357. Recoil was mild, brass ejected without any sticking, there was no visible evidence of flattening on the primers- The weapon is used was an old Ruger security six, with a 6 inch barrel. There was no barrel leading or lead build up in the forcing cone.

Thank you again for all your comments.