Actually, there isn't a set number. The OAL in the manuals is usually a SAMMI maximum intended to keep rounds moving freely in a magazine (or from protruding out of a revolver cylinder and jamming it). It assumes round nose or spitzer or whatever the most basic standard shape bullet for your cartridge is. As you can imagine, if you had a bullet with a long, skinny tapered tip, it might chamber and shoot safely, but not fit in the magazine or cylinder. Rifle shooters run into this with VLD bullet profiles all the time, and can often only load them singly.
If your magazines have a rounded profile on the front edge, a flat nose bullet like a full wadcutter still may not fit even though it obeys the SAMMI OAL number. Wadcutters sticking out that far will also run into the throat of the rifling before being fully chambered. They are often seated flush with the case mouth for this reason, and not to standard SAMMI OAL.
As you seat a bullet deeper, it consumes powder space. For a fixed number of grains of powder, deeper seating means higher pressure because the powder is in that smaller space. Any time you seat deeper, you need to work your load up over again.
If you have a new bullet shape, take an old case with no primer or powder, seat it as far out as you can, with no crimp. See whether it drops into the chamber to full depth? Keep putting it back in the press and moving the seater down until it does just go in. When it does, you can work up a load and shoot it by single loading for sure, except in a revolver where it also must not be longer than the cylinder.
After seeing that it chambers, if you also want it to feed from a magazine, check to see if it fits? If it is too long, start nudging it down further until it does fit. Try feeding this dummy round a few times. If it feeds OK, you have found a new maximum OAL for that bullet in that cartridge. If not, you have to seat still deeper until it does feed, and then you will have your OAL for this bullet in this cartridge. (Occassionally a bullet shape won't feed at any depth without modifying the feed ramp or magazine fingers, etc. At this point you need a gunsmith or a different bullet.)
Now you are ready to set up to make your load. You will find that bullets seated and crimped in one stroke stick out a bit further than those nudged into place in steps. If you set the die up to crimp, you are going to have to change the seating stem setting anyway to achieve your new-found OAL.
Use your new maximum as a maximum, and not as an average. If your bullets go both above and below this number, those above it may fail to feed or chamber properly. So nudge the seater down as much as it takes so the longest round you ever get is at or below the maximum by a couple thousandths.
Now go work up your load, and be safe!