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.
