There are several ways and none of them are a secret. Something I started doing last year, this is very helpfull for Saitos because the factory sets them so fat, works on all glow engines though. I have the engine all warmed up and with the engine running and the glow driver on I remove the glow driver, if the engine drops in rpm I put the glow driver back on and lean the low end then remove the driver again, it the rpms drop I do it again. When the rpms no longer drop I open it up just a little bit, low end is adjusted.
TACHs, I have always adjusted by ear, no problem. I had three tachs and all were total crap. Then I got a TNC Tach for prop testing, does a wonderful job for that on gas or glow. Then I started using it on my glow engines just because. It really does work a lot better then my mark one ear ball on most engines. No, I don't use it all the time but if I really want to see what an engine is doing then out it comes. Not all Tachs are created equal, I have three junkers under the bench but a good one can be a god send and come in handy. I now use it a lot more then I once did, not really needed but I can really dial in an engine and see it happening.