The purpose of the mainspring strain screw in a Smith & Wesson revolver is not to adjust the weight or strength of the double-action trigger pull, or for that matter the single action pull as well. What it is supposed to do is release the spring's tension so that any strain on the hammer stud (the pin the hammer revolves on) is released before the side plate is removed.
In the worst of cases fooling with the screw can result in misfires, hang fires, and even a broken spring.
You can lower the weight of both trigger pull systems by replacing the stock mainspring and rebound slide spring (that acts to push the trigger forward after it's pulled) with lighter ones made by after-market suppliers. However going too far with this is likely to result in misfires and short stroking the trigger (trigger returns forward too slowly and jams the action).
You would be much better off leaving the springs as they are, and practice double action shooting (including dry firing with snap caps) until you have built up the strength to handle the pull as it is. Any other option will reduce dependable ignition to some degree.
