Been there, Done that, blew out plugs too.....Since last summer I have been through this situation as well. Trex 600N Pro. Os50 Hyper. Too lean takes it out for sure. My needle range is about 1 1/4 turns out to 1 3/4 turns out through August's @ 80 degrees to January's 20 degrees. We are at 1300 ft above sea level. Your setting will vary.
I blew 2 motors and about 15 glow plugs.
Scratched my head when I had a perfect flight/landed and tried to fly again only to find the Glow-Plug was burned out. I guess the coil breaks in flight and keeps glowing from combustion and allows the engine to run. Once it shuts off, a new plug is needed.
I hadn't flow Glow aircraft since the early 90's and this was the first glow heli I had flown. I had nobody close by to help me in the beginning so I just keep trying to learn. I burned up 2 50 Hypers with holes in the piston and many plugs.
I settled in on a tune up flight each day before normal flying. I set my needle where I knew it would be close.(for me at 1 1/2 turns out. For engine safety I hovered - making sure I had a lot of smoke, then flew around watching to see if the smoke diminished. Last but not least I perform a long climb out while gradually increasing blade-pitch to see if the heli bogged. When It bogged I knew it was lean and sat her down and turned 4 clicks out (richer). This was the way I learned to tune it the Hyper so it ran right. Without fail this process gets my hyper tuned so I can fly.
Now, I always turn 4 clicks at a time. When I get it tuned in close I click 1 click at a time to fine tune it. (Arguably) the needle has 32 clicks per 360 degrees as I counted. 8-clicks per 1/4 turn. 4-clicks to the 1/8 turn. When I click 4 clicks twice I keep a mental note that it is a 1/4 turn difference. This way I could keep track mentally of the setting. I have written this down for my records too. I always close the needle and record the setting at the end of the day as well. This is the "days" setting, or the right setting for that day. I go through this procedure because I am critical on how my engine runs. I love to run the engine as hot as I can to give maximum power. I have learned a lot from this process and my records. The most important thing I learned is how much the final setting varies from day to day. I never would have thought.
Hope this helps you.
Foreseeing My Flybarless Future!