I got myself one of those adjustable buttplates that allow one to adjust the length of pull from the shoulder.

Is there a "proper" way to adjust it, or is it just something that I have to determine what feel right?

TIA
JDP

The way I was always taught:
Rest the buttplate in the crook of your elbow, muzzle pointed at the ceiling.  Correct length of pull is when the first joint of your trigger finger will comfortably touch the trigger.

A good starting point is to grasp the pistol grip and see where the butstock comes to in regards to the inside of your elbow when your are is bent 90*. The ideal lenght will be when the buttplate just touches.  

If your bench shooting, you may want it to be a bit longer.  If your using it for snap shooting, then you may want to go a bit shorter. But, it all comes down to how the LOP feels to you.

When in doubt I'd go just a tad short on LOP if you ever plan using the rifle in cold climates when you have lot of clothes on.

LOP is most critical with shotguns because it is necessary to mount the gun in a fraction of a second to shoot birds and/or clays.

Most rifle shooting I see people take their time shouldering the rifle, aim, then carefully squeeze the trigger.