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.
"Why is is called Common Sense when it seems so few actually possess it?"
Guns only have two enemies: Rust and Politicians.