If a person knew the thickness of the head of the Allen screw and the pitch of the threads couldn’t he set the length by just calculating the turns out and save the disassembly?
The best way to get there (better than measuring) is to first get set your regs set to a decent spot. You can use one of the 13 shot charts I've provided as a guide, or you can ask a buddy what they use. Remember, those regs may need 5-10 shots to settle into a final resting place after adjusting. Start them looow (70 bar for 25gr and 90 bar for 34gr). I like to hunt around in 5 bar increments. And remember, a lotta people out there massively over-regging their guns. Forcing everything to work harder (regs & valve) definitely has its place as a tuning tool but only by a little, and when you're 9/10ths of the way there.
Next, back way off the internal micro HS tension (CW) to get ya down into the 800's when on a PW setting of #7. Slowly work the velocity up (CCW on the internal HS adjuster) by a 1/4 turn at a time to bring you into the desired velocity range. From there, fine tune the IHS adjuster up and down in 1/8 turn increments (then less) to stabilize and compress the ES & SD in that range. Shoot a string. If the string has an ES closer to 20 than 10 or more importantly has an SD closer to 6 than 3, up the regs by 5 bar at a time and repeat. This will have a tendency to further stabilize your HS settings, but not always... This is why I like to start low on the regs and work them up in 5 bar increments once a decent reg/HS place is located. Finally, if you find yourself with a good chart but want a tighter SD, first try flattening those alligator teeth with a tad more or less IHS. Then try by following up with a tad more reg. You'll get there.
Also, do ALL your test work within 10 bar either side of 200 bar, cuz if you've got linear strings around 200 bar, then you'll also have them at 250 and as the gun falls off the reg. The same isn't true of beginning at 250 each time.
Also, don't be afraid to use the macro PW adjuster around #2 #3 #4 with 25gr pellets rather than back waaaay off the IHS micro adjuster to get ya down there on #7. I used #2 #3 #4 with some excellent results and it's much easier going back & forth between tunes this way too. There's no need to ever turn the internal IHS adjuster more than 3-4 turns when hunting around for sweet spots between 500mm and 700mm tunes (25gr to 40gr) if you utilize the macro power wheel properly. I'm much more in favor of that than putting it on #7 and moving the IHS a mile to get velocity down to where it works with a 25gr and the 700mm.
The above is how I do it, and you'll find those 10ish and 2ish ES & SD's every time doing it this way. It's fast, easy, repeatable, effective, and no disassembly required!
Have fun and don't be afraid to experiment by going different ways than what you've heard works well!
Steve