Hornady bought the old Stoney Point OAL measuring device. Scheels sells them and the modified cartridge cases you need. Just follow the directions exactly and you'll know the OAL that places THAT bullet on the lands. Remeasure for different bullets.
How far off the lands? .03" is a starting point, if the rifle's magazine is long enough. Testing different OAL settings will tell you what that barrel wants.
A caution, and it's important. If you seat the bullet further out than factory length, you have increased the powder capacity of the case a tiny %, but you've reduced the freebore (distance the bullet jumps before meeting resistance) by a large %. Don't assume you can use the same powder charge with an extended seating depth!
Your best friend is a chronograph. After seating the bullet longer than factory spec, return to starting loads and slowly work up to whatever velocity the powder or bullet maker's reloading manual says you will reach with a maximum powder charge. STOP. You have reached max permissible chamber pressure, no matter how much power you have in the case. Now use the powder charge that's most accurate.
My 338WM with longer than factory OAL reaches the published velocity with 3 grains less powder than the powder maker says I need for that velocity. My 30-06 takes 2 grains less. Both loads are quite accurate for over-the-counter game rifles (sub 1" five-shot groups @ 100 yards).