Canabian,

If I understand your question correctly, in most diesels there is a contra piston that is moved down by the screw. When you turn the screw out, compression moves the contra piston up to the screw while you flip the engine.
Instructions that came with one of my diesels said to turn the screw in until the contra piston touched the piston at the top of it's stroke, then back off 1.5 turns. That established a starting point.
Most diesels come from the factory with compression set to a running position. If you hand a diesel to a friend, invariably he will mess with the compression screw...unless he runs diesels also.
What diesels are you referring to, someone may be able to offer specific hints for that engine.

George