All published "maximum" loads don't necessarily relate to pressure or safety!

In this case it is to keep velocity, and consequently leading, down to reasonable levels. While there will be a difference in velocity between .38 and .357 with the same load it really won't be that significant with most faster burning rate propellants.

Why are you wasting time and money loading Trail Boss in these calibers? There are many other propellants far better suited, and more economical, than TB.