You don't want to look at the data for H110 in 357 Magnum, if you look at enough different sources you can find a 3 grain range between the maximum charge level.

Generally I try and find an "area" where 2 or 3 different sources overlap and start at the low end of that range. Then it's a matter of being observant for excess pressure indicators and how well the ammunition functions in a semi auto. I also test for Position Sensitivity with Revolver calibers because I learned that lesson the hard way, long deep cases can create Position Sensitivity issues with some powders. Once I find a point where function and pressure indicators are all green I'll then start testing for accuracy and try and tune the load for the best balance between Accuracy, Function, and Economy. BTW, that means that if a 5.4 grain load produces the same accuracy as a 5.7 grain load and both function well my choice will be the 5.4 grain load. Yeah, 0.3 grains isn't a lot but over 2 or 3 thousand loads it does add up.