Max 'book' COAL is determined by what can normally be fed through the action without jamming, eg in the magazine. That's the longest it can be and still feed properly in a repeating rifle.
Max 'practical' COAL varies with each rifle as it depends how far into the barrel the bullet can go without touching the rifling. (Some want it to touch, others want it not to. That's another issue. It cannot in any case be longer; seat it further and the rifling will generally shove it back into the case, which is bad. (There's a caveat to that. The shape of different bullets will allow some to go further into the barrel than others before contacting the rifling. The max practical COAL thus differs for each rifle and each bullet.)
LUTNIT's point about theoretical depth of seating is reasonable. Look at the neck of your casing against a bullet. There's no point in seating it so far in that the bearing surface of the bullet is below the neck. (There are other problems with that, but bear with me.) On the other end, seating it so shallow that it's not stable or secure is obviously foolish.
Seating a bullet too far in can raise chamber pressure, so particularly when you're learning, it's better to seat a bit long than too short. The problem is worse in smaller rounds, generally, but a bad idea for any round. Obviously a long bullet will have more protruding than a short one. It's how much volume inside the loaded case that's full of bullet (vice propellent or just air) that matters.
The bullet wants to be at least far enough in that it will be held firmly, yet not so far in as to raise pressure to a dangerous level, nor yet again so long as to be pushed back by the rifling. It gives you a range of seating depths, if you follow my drift.
So...
Start off with a reloading manual. One of the ones by the major companies like Lyman, Sierra, Hornady, etc will have a good coverage of bullets and powders. Start loading at the bottom end of the recommended loads and go for a 'ladder test' - load five rounds, then increase the load by a bit and load five more. Repeat. You wind up with a series of slightly-different loads, with the amount of propellent being the only difference. Go to the range and test fire. Watch for signs of high pressure (again, your manual will explain those). When you start getting them, stop and recycle those rounds and all others with more propellent in them. (Recycle = pull bullet, return propellent to the jar, etc). In any case, approach the manual's recommended max load with great caution.
You will get one, maybe two loads that produce better groups than the rest. Start playing with the amount of propellent in smaller increments until you find the best one. That will probably be accurate enough for most purposes. For maximum accuracy, you then start adjusting depth of seating (you also start treating each case like a princess, but that, too, is another story.)
