I use a Allen key. Holding the short end I crank it as best I can holding on the short end. Not very scientific but seems to work. Hand strength dependant. Ymmv.
A little trick I learned also. Is these screws don't come with loctite of any sort on them from the factory. Seeing how you will likely want to take it off often and swap guns n stuff, you don't really want to use a permanent setting kind of glue. So I take a dab of E6000 fabric multi purpose glue and put a tiny dab on screws. As you shoot these love to rattle loose. This type of glue is clear and rubbery when it dries. It's just gummy enough to keep them held but forgiving and won't lock anything up. They may make a better proper version of this glue for this job but it's just what I've found to work that I had laying around at the time. Also press the optic forward against the picrail as you tighten it. Having it pressed on them keeps it from shifting under recoil. Anyways a little soft glue makes it so I don't have to rely on a crazy tight torque and possibly strip things. I wouldn't use this on things that gets hot tho.
I found one at walmart.com that was inexpensive (If I remember it was $30 or less) and works great. Brand is "Autlead" I checked the inch pounds against a 1/4" torque wrench that I have and they were the same so I guess it's fairly accurate. Has a "T" shape so easy to hold and torque properly
Not cheap--but CDI torque products are excellent. No spring, so calibration is not required (downfall of most wrenches/drivers). Mine seems very repeatable, and they have a big range of torque values available.
Purple threadlocker is good too if you feel the need to threadlock. Minimal resistance when you go to remove it.
I use a TEKTON 1/4 drive torque wrench. Model number TRQ-21101, 10-to-150-inch pounds. Online they are $52. If you need a set of 1/4, 3/8, 1/2-inch drive, they have the set at $148.00
I just got too nervous trying to guess at 20 inch pounds applied evenly on all ring screws with $1,000 plus scopes. It's a fine line between 20 inch pounds and busted internals.
Yeah I'll have to get some tips when ya come by on that. So far all my stuff is red dots etc. It hurts to say it, but they're all 400 dollarish ones roughly. Hard to believe that's just for basic budget stuff mainly. Optics are definitely an expensive game. When i set up a long range one I'll have to definitely upgrade the Allen wrench!