Dear Pooya, eventhough I don't know whether this principle applies to shoulder joint or not. normally for ACL injury in knee joint will increase when the agonist-antagonist ratio deviates from 0.80-1.25 range (1.00:0.80). I think this applies to any joint.

In my experience the strength and balance of shoulder muscles is reflective to pain and the ability to stabilise the scapula at the trunk! So it is worth to look first at the deeper scapula muscles (trapez. asc./ lat. dorsi/ rhomboidei) in the balance to the front muscles (pectoralis) and upper scapuala muscles (levator scap.) and set this in relation. Very interesting approach! Let me know the results. In this relations we have only "circa" normes

Hi 

Normal force is relative. Best normality parameter is compared with the contralateral side (if intact)

Hi

The answer depends what you are looking for? Do you mean IR/ER ratio? Otherwise strength is affected by many components.