Cartridge OAL is most important with respect to magazine length and then maximum seating depth (follow quicksdraw45's advice here). In a single shot rifle, magazine length is not a consideration, so you get to determine the best lenght to use for your rifle/cartridge combination.
You need to learn to measure the length of the cartridge from the base to the bullet ogive - this is the important measurement for developing best accuracy.
The old time method is best - size just 25% or so of the neck of an umprimed cartridge, seat a bullet just far enough to get it started straight, then chamber the bullet. The neck should be sized to provide just enough neck tension to permit withdrawing the cartridge from the chamber without pulling the bullet out of the case if it sticks in the rifling lands. Two methods can be used to treat the bullet before chambering to make the origin of the rifling easier to see - the first is to polish the bullet bright with steel wool, the second is to darken the bullet with a marker or soot. The rifling will leave small rectangular marks on the bullet - this indicates the origin of the rifling you will work from.
Now, after the bullet is fully seated against the lands by closing the action, withdraw the cartridge and measure the length from the base to ogive. Get the 6 sided tool from Sinclair to go with the 6 inch calipers you probably already have if you are reloading. This length is your starting point for experimentation. Unless many other shooters are getting best results with a particular bullet seated far off the lands (0.015 inches, for example), you should start with the bullet only about 0.001 to 0.002 inches off, then try 0.004 or 0.005 off, and so on to about 0.010 to 0.015 inches off. If you have a good handloading mentor to supervise, you might even try a load with the bullet just touching the lands (this is where most benchrest shooters seat to), but be careful because high pressures are easy to generate (for damn sure don't use a maximum handbook load to start!).
If you don't have some sort of comparator to measure the length to the ogive, you can measure the OAL as an interim measure, but this isn't very satisfactory since bullet length varies - greatly in the case of HPBT's.
One other detail to note is how fast the chamber throat erodes as you shoot the rifle. In good stainless barrels shooting .223 Rem, the throat will erode about 0.001 inches per 10 rounds. This means that you will need to adjust bullet length as more rounds are fired. [The good news is that the throat erosion slows, sometimes.]
After you go through this exercise a couple of times, it will make perfect sense and be no trouble at all.
