Good question, despite it seems to be bagatelle.
I think everything depends on the glowplug washer. Many original factory washers are problematic.  It must not be too thin (less than 1/32"), too thick (more than 1/16"), too soft (pure aluminum), too hard (Dural, brass, iron, steel).  Let me not explain, why.
My bet is to make / order some dozen of pure red copper* washers 1/24" thick, with precise 1/4" inner dia. and not less than 3/8" dia. outside.
This is not a task to machine it with the most primitive lathe.
Tighten it with a 4"  "T" key, bravely. (Cannot estimate, how many lb. fts)
Istvan

*where to find red copper rod: bigger soldering iron's spare "iron" used to be made of red copper

Just for the record this was a new Enya No. 3 with a new gasket. Checked before I flew today and again got 1/16 turn tighter. Flew a few flights, engine was back to normal starting procedure. Checked again after second flight and still got another 1/16 of a turn. I assume the new gasket keeps compressing will see how long this goes on. I am looking at a 1/4" torque wrench from Harbor Freight item #2696, Pittsburgh, $19.99, 20 - 200 in-lbs. I don't know if this is enough by seems like is should be. Will try it and report back.

Best,    DennisT

Just for the record this was a new Enya No. 3 with a new gasket. Checked before I flew today and again got 1/16 turn tighter. Flew a few flights, engine was back to normal starting procedure. Checked again after second flight and still got another 1/16 of a turn. I assume the new gasket keeps compressing will see how long this goes on. I am looking at a 1/4" torque wrench from Harbor Freight item #2696, Pittsburgh, $19.99, 20 - 200 in-lbs. I don't know if this is enough by seems like is should be. Will try it and report back.

Best,    DennisT

  I think the last thing you want to use on a glow plug in a long handled torque wrench, even a 1/4" drive. I think you will easily over do it and maybe even break something else with all that leverage. Some where along the line I remember seeing an add in the magazines for a small, in line type torque wrench that may be what you want and I can't remember what the graduations were in, maybe newton meters. After doing this sort of thing my whole life and earning a living using tools for the last 40 some odd years, I think even 20 inch pounds is not necessary. I do as Brett suggests, then like you do, check after a couple of flights. I don't try to turn it any more, just make sure it's snug and let the washer do it's job. All the extra torquing may strip a plug hole. I have never had a glow plug come loose except for Cox plugs, and it is one of the great mysteries of the universe in how they manage to come loose like they do. Maybe its the aluminum plug in a steel cylinder and the large diameter and thin copper washer that has something to do with it.
  Type at you later,
   Dan McEntee

I checked the OS instruction book for the 46LA, and they just say "make sure it's not cross-threaded and then tighten firmly".  If it was at all critical I think they'd give a number just for CYA purposes.  It certainly matches my experience, and everyone else's suggestions.

Just as an FYI, this page recommends 57 inch-pounds for a 1/4-28 aluminum screw (no word on what it's going into), which is a higher torque than a 1/4-20.  I suspect (but will bow out of the discussion if challenged by a machinist or mechanical engineer) that you can figure that's a good "do not exceed" number for a glow plug.  It's probably freaking tight, actually.

Hello When ever I get a plug that does not screw in 'cleanly' or feels loose in the treads or 'continues' to tighten I pull the plug and check the threads and the sealing surface on the head. With second hand engines it is not uncommon to see damaged threads/surfaces as people insert glow-plugs without washers and ruin the surface or use a short reach plug and over-tighten . Have seen new plugs with badly cut treads too (DC plugs were the worst but long gone now) Pulled a new plug out of the packet today and it was rusty and presume very old stock but brought in the last year. 
Regards Gerald