Hi Craig,
Usually the larger screws are for adjustment and the smaller ones to lock it off.
The screws work on a push / pull system. The locking screws push on the back of the mirror cell to lock it in place. There are springs in there too which push the cell to help when adjusting.
Its the pulling screws which you adjust. Its easy to tell which is which. The locking screws move up and down when you screw them in and out. The adjusting screws always sit flat on the back of the cell. Also the locking screws will feel quite loose until they actually lock against the cell. The adjusting screws always feel like they are under tension from the springs.
Does this make sense?
So the procedure would be to loosen the locking screws. Make your adjustments with the other screws then lock it all back up.
Hope this helps
Cheers!