Assemble a dummy round and seat the bullet very long with just enough neck tension that the bullet doesn't move freely inside the neck. Chamber it. It wil hit the lands and push back into the neck. Eject it, measure it, and then adjust your seating die to it. Then make micro adjustments deeper to keep the bullet just off the lands. THAT is your maximum OAL according to chamber. Now, max OAL according to magazine may be somewhat shorter.
What he said but here is cool tip. After neck resizing, use a dremel tool make two cross cuts into the top of the neck using a carborundum blade (very thin).
The bullet will hold nicely. Once you have determined the best OAL, save the dummy bullet for future adjustments. You should have one dummy round for each gun. Finding that sweet spot really makes a difference in accuracy. I made about 5 different loads (3 each) of OAL and simply bench tested for accuracy. Each load was .002 difference in OAL. My end result was a bullet that was too long for my magazine but that's the way it goes.