I will answer you to the best of my ability.
I'm NOT referring to the original Factory length here but the length I have finalized them to. Factory Strain Screw lengths are 100% reliable but to me they are just way to excessive and make the pull too heavy. There IS a happy medium IMHO! Off the top of my head I do not know the Factory length because I have never measured them and at this point I doubt I have any revolvers that have the factory length screw still there as they are usually too long for a smooth action as I like it - especially the newer ones. It all depends on several things and just so you know I never even bother to measure it. I go strictly by "feel" and performance but if one was to measure it, a Calipers reading in thousandths of an inch would need to be the tool to do so, NOT a rule.
Each Revolver is a bit different due to the specific springs in the gun, how old and worn in the gun actually is, what spring weights they installed, how your Revolver's specifications are (end-shake & tolerances) and exactly how far your Hammer retracts in DA mode.
In general, I know by feel just where to set the length to but that is very hard to convey on-line. Your best bet is to buy a few extra screws and experiment. They are cheap enough to do so and you can custom tune them for the smoothest or lightest pull you can but still have 100% reliability & good primer hits. I always use CCI primers as a "litmus test" as they are generally known to be the hardest of the primer brands. If your Revolver functions 100% with CCI's then you should be GTG. Once you have done this to a few Revolvers, you will sort of get the feel and know in short order within a few thousandths where to stop - without even measuring. If you do go to far and your action is not reliably setting off all primers, just toss the screw and start over.
When I work on Strain Screws I also round off the end just as they are form the Factory, and sometimes even re-blue them making the job look professionally done. NEVER just leave a Strain Screw full length and backed out. This leaves the screw loose and will work itself out under recoil. Always make sure it is tightened all the way. Loosening it under fire CAN be TEMPORARILY done JUST so you get a sense of how much to safely remove. Once you get that sense, remove your metal, reshape tip and try. If OK, you can re-blue it if you like. I also suggest leaving the originals alone and place them in the box with a note that they are the Factory original Strain Screw and do your thing on a replacement screw. AGAIN....... buy a handful so you never have to scrounge for one if you go too far!
If you simply want 100% reliability and are amicable to s stiff pull, just install a new Factory screw and you are GTG. If you want both reliability and performance, the procedures above are the way to go IMHO.
ADDED: I have also noticed over the years that Factory Strain Screws are NOT all exactly the same from year to year and from S&Ws supplier to supplier. They have varied a bit and so has the shape and length to some extent. Another reason to fit it to your individual gun.
