YUP find where your bullet touches the lands>>> Find how long the clip/mag or what will work in the rifle>>> then figure out where you wanna start like 20 thousands off the lands.
Book COAL is arbitrary, trying to create a rule of thumb off of it like is being discussed would be a mistake IMO. If you're going from 2.73 to 2.80 but you're not jammed in the lands until 3.00" COAL, it doesn't matter and I'd make no additional considerations. If you're going to be kissing the lands at 2.80 COAL it does matter and your pressure will likely increase when loaded longer.
Think of load manuals that list max charge weights for a list of similar bullets, I promise they dont all hit max pressure at the exact same charge and wouldn't be surprised if COAL varied a bit between them when tested.
Measure my mag internal length and load a little shorter than that so it doesn't bind and I have enough bullet in the case to have bearing surface in the neck for the full length of the neck. I don't alter my starting powder charge and I'll usually do 0.5gr increments then when getting towards book max I'll go down to 0.3gr increments.
But determine your max charge by what happens on the bench. Thinking about what max might be is fine but if the bolt gets sticky at the range I'm not going hotter.
Now that a length/ seating depth has been determined its a good idea to check it in the rifle first before jamming a bullet in the rifling and spilling powder ripping it out. I take a case thats been fired in that rifle and not resized. Run it up into a lee factory crimp die just enough to bring the mouth in a little for tension add a little blue locktite in the neck and seat a bullet long. You can put some black sharpie on the ogive too. Then get the case rim under the extractor and gently chamber it in the rifle. Leave it alone for 20minutes then gently run the bolt and extract it. Measure COAL and CBTO (there should be little lines in the sharpie from contact with the rifling) and you have determined length to the rifling with that weight and model of bullet. As long as this length is longer than your intended COAL you can proceed as planned. I go for 20thou of jump if I'm close to the rifling.
