First, take a look at other cartridges and you will quickly see the differences aren't limited to .243. Books vary and that creates confusion. Having more than one manual is not the aid many "old hands" seem to think, especially so for noobs; we too quickly forget how confusing it all was at first.
Then, as Shot 1 suggests, your rifle is different from their's and their's are different from each others. If we all used the same firearm we would all get the same results. Whatever their pressure level may have been during their tests yours won't be the same as either of theirs.
Pick your book of choice and follow it. "Start low, only move toward max unless or until...." etc. The way we develop a load is much more important than what any book lists for a max charge. There is little point in starting low if we are blindly going up to max anyway. Yeah, book data is USUALLY safe but not always; those who proclaim loading books are "lawyered up" are wrong. I know one guy who got blown primers - WAY HOT! - by starting with book mid-range charges of Varget in a brand new Winchester M70 rifle.
Knowing what we're doing and paying attention to what's happening is the ONLY way to stay safe. No book, or ton of books, even if they come from the specific powder or bullet makers, nor any "Pet Load" source, can take us by the hand and carry us to that end. Noobs can't possibly know all they need to know about pressure signs so staying conservative until they learn is the only intelligent way to go.
