What I did before I bought a Stoney Point gauge:
Take a case fired in the rifle in question. Do not resize it.
Apply heavy thumb pressure to distort the case mouth enough to have some friction on a bullet, but not so much it won't go back in the chamber.
Put a bullet in the case neck as shallow as it will stay in place with careful handling.
Gently chamber in the rifle, gently extract, the bullet will have been seated by contact with the origin of the rifling.
Measure OAL.
Subtract .030" for a starting OAL.
Load and shoot.
How can it damage anything? You are pushing a copper bullet against steel rifling with fingertip pressure. When you shoot, you are pushing it in with 50,000 psi.
You might stick a bullet in the leade if you don't get the case neck to the right tension. So push it out with a cleaning rod and try again. Try again anyhow, the method is not real precise and some replication will help.
The two Savage 12BVSS-S .308s a friend and I are working up have rather short throats. A SMK at 2.80" is touching rifling, We started out testing at 2.775. But that will not translate directly to the Hornady bullet with secant ogive and plastic tip.