Instead of expecting to get higher velocity you are better off revising your expectations to what is reasonable. 1258 FPS from a jacketed 158 gr. bullet from a 6" barrel is about all that can be expected, and I say this after many years of chronographing .357 Magnum with many different propellants.

Depending on your gun, not the powder lot, you will see between 1200-1350 from a 6"barrel revolver. The only way to improve on this is to change to a cast bullet and you can gain about 100 FPS by doing nothing else.

Look at it from a practical standpoint. With your load and a 50 yd. zero range your drop at 100 yds will be ca. 5.4". Even if you could get velocity up to 1350 drop would still be ca. 4.7". The difference, .7", is about the diameter of your index finger if you have normal hands. Do you really believe this is going to make any practical difference at 100 yds? And at this distance bullet energy is down to ca. 400 '#, not even legal for Deer in many states!

In spite of the fact that animals as large as Elk, Polar Bear and Moose have been killed with a .357 Magnum it wasn't done at ranges where bullet drop was a factor. .357 Magnum is a marginal big game cartridge from a revolver at reasonable ranges, under 50 yds. This applies to all revolver cartridges!

If you want a 100 yd. gun buy a rifle, don't try or expect to make a 6" .357 Magnum into a 150 yd. Deer gun by magic loading, it isn't going to happen, isn't fair to the game and isn't good sportsmanship.