Most bullets I don't mess with seating depth as I seat to crimp my bullets into the canalure.
Other than that, work up powder charges as you would for a rifle noting which is most accurate. For loads under 7 grs I work up in increments of 0.2 grains. For loads greater than 7gr I work up in 0.5 gr increments. If I'm not happy with the accuracy I'll try a different powder. Starting with a bullet that is sized for your gun, and with a powder known to provide top accuracy is the best starting point.
A good revolver off the bench with a good load will group 1 - 1 1/2" with irons at 25 yds, a very good scoped revolver will group 1/2" at the same distance. I've seen groups tighten up as you reach the sweet spot, and open up as you pass the sweet spot.
Not to be a smart ass but the best way to work up accurate loads is start with an accurate revolver. I've had a revolver that make me tear my hair out trying to get better than 5" at 25 yds, and my scoped 480 was shooting 1/2" groups at the same distance. Having an exceptionally accurate revolver will teach you what you are capable of with a revolver, but then you'll get frustrated with your problem children guns. No amount of load work will turn a 3" gun into a 1" gun. So you need to have realistic expectations.
Try some wadcutters with known good target powders and charge weights in your M19 as a baseline. While mechanically I would think 357 brass should produce better loads than 38 sp brass, the added powder space in the 357 is not conductive to accurate loads with light loads so stick with 38 sp brass for mild 38 loads.
