The concept that some old military chambers and sporting rifles will not develop the same way as some BR rigs or match guns will, is the only difference I have with some of the posts here, but it is worth pointing out that what I will add isn't really in conflict with the previous posts when you step all the way back and look at the problem as a whole.
The clubs I shoot at have a limited membership headcount, but even with that we have a constant flow of rookies who ask similar questions every year. Depending on who they ask, they can get easily confused. My opinion is that is because some of us have been at this so long we forget how confusing this can be when all you do is get told what to do but don't know enough to understand the context or the reasons.
In many load development arenas, the chambers and bbls are not to the standards of BR rigs or match rigs. These may include AR Service Rifles, the "Wooden Guns" of the Western Games, antique hunting rigs, etc.. In several of these instances, the feed reliability means the bullet tip styles and magazine feed reliability issues can dictate that the load developer must to their homework and keep those issues in mind. Magazine length restrictions can limit bullet choices and seating depths on both the long end and short end. So in a way, I still agree with many of the comments, but for different reasons than have been pointed out to the beginners in the forum audience.
A starting point for seating depth can come in two forms. One can be a whole wide range to go explore, and the other can be narrowly constrained by the design of the rifle.
Keeping in mind that in a sense these reliability requirements are yet another constraint, as often as not the shortest path is to research the seating depths that make feed reliable, and then start with charge ladders. Same advice but for a different reason.
Once the operation of the gun is reliable for function or rapid strings, and the charge ladders point to promising wide nodes, then the smaller test ranges on charge and seating depth can be done with finer increments and larger samples to get to the point with what would still be the most efficient round count. So not really a conflict with what has been advised, but to highlight issues that drive the reason to check seating depths and charge weights with a particular range and order.
The questions to ask yourself up front... does this rig require a wide sweep range of seating depth and charge, or is there some good standardized starting point that can help minimize the search. For beginners, we can offer good starting "pet loads" that go with some standardized rifles. The example of Service Rifles from CLE or WOA versus the wide open question of all ARs comes to mind. We can give a narrow search window of seating depths and charge weights for CLE or WOA, but that testing looks different when you throw the discussion open to the whole wide array of all other AR bbls, twists, and chamberings. We can even say with some confidence where you will end up finding seating depth nodes and which one might last the life of the bbl for the 600 yard line. But that is because we know how standardized that particular context can be built. The needs of an AR Service Rifle are just one example of a context. The answers would be different for a Palma gun or an F-TR rig.
In the end, getting the context laid out is all I am adding. The fact is, sometimes you do start with an assumed starting charge and a seating depth sweep, and sometimes you start with an assumed seating depth and sweep for charge first. The answer can depend on what we already know about your gun and context.
