IMO, the correct, old school, B&W Precision Shooting, short range benchrest definition of jam is the length that a bullet would be pushed back to when seated long and chambered, using the neck tension that will be used for the actual loads that will be shot. You can either set your seater to seat bullets to that length, or get there by loading a little long and finishing seating when you chamber the round before shooting it. So sawacs...from my perspective, you are right on the money. Now that we have that settled, on to the original question... or something closely resembling it.
When I am working up a load for the first time, I establish jam length, and then back off about .003 so I will not stick and pull a bullet if I unchamber a loaded round. Then I pick a safe load that is well below my intended velocity, and shoot a test, trying to shoot all shots in the same condition, on a day when the wind is easy, holding center, on a single target, shooting over flags. For something like a PPC I shoot one round per load, after fouling the barrel with a couple of shots using the same powder, moving up .3 gr. per step, to the point where I feel the bolt lift start to become more than I like. For larger and smaller case capacities I vary the weight increment. A Hornet would have .1 steps, a .220-250 perhaps .4, and everything larger .5. As I shoot the test, I note how the increasing charges influence where the shots impact the target, paying particular attention to clumping , when two or more charges shoot close to each other. After I get to my slightly too tight bolt, or what ever pressure indication that I have chosen for the cartridge being tested, I pick my best looking cluster and load up a two round test of the middle charge, and shoot it as possible so as to have the best chance of getting off both shots in the same condition. If the result is good, I load more and shoot more test groups of larger numbers of shots. If it is not, I consider what change makes the most sense to try, load up a couple and try again. Usually, with a good rifle, I will get predictable results, but there have been times when I have had to drop back and punt. Bottom line, I start low and work up, seating near jam, so that I have a worst case situation, anything else that I try in the way of seating depth will have less pressure. I have not found that there is a significant pressure difference between near jam and jam. Once I have my charge limit, and a possible good charge, then I start experimenting with seating depth, usually all into the rifling by some amount. If I missed a lick anywhere in this, feel free to comment.
.
