It looks like people are looking for that magic number again. Just Google magic number and you will get nearly half a billion results. The problem is that every engine is as different as each operator. I can't condense 50 years of operating glow engines into a few sentences, and am willing to bet nobody else can either. But I do have a few general observations. Most problems are with the set-up of the fuel system. Most operators tend to run their engines too lean. Most fail to monitor the health of the engine over time. Here are a few things that you should be aware of if you are running two cycle glow engines.

Fuel system
Fuel must be very clean, as very small particles of anything will plug up the needles changing the mixture.
Lines to the tank must be as short as possible, make sure there are no leaks.
Tank height should have the center line of the tank no lower than 1/2" below the center of the carb where the spray bar goes through.
The clunk inside the tank should be able to touch both the top and bottom of the tank without touching the back. Fuel line grows in length over time once exposed to fuel, so start with the clunk about 3/8" from contact with the back. Use a two line system if possible.

Running too lean
Pretty much self explaining, but if everybody is doing it then nobody seems to recognizing the problem. When on the ground, an engine is under a much different load than when in the air. Most sport models with fairly clean (low drag) airframes will have the engine unload 1500 to 3000 rpm over the ground rpm. This also requires more fuel. Running light loaded props helps, especially with engine with low time on it. But for a broader needle range which is much easier to tune, run a lower pitch prop for a given diameter. While you might thing that you will be flying slower as a result, often the air speed is nearly the same due to increased rpm.

Condition of the engine
I have often sat on the sideline and just listened to engines in the air. Usually in a few seconds you can tell the condition of the engine, from being too lean, to even the condition of the bearings. A bad bearing has a very distinct sound that most seem unaware of. Complete bearing failure will do enough damage to a engine, that is not economic to repair, so before running an engine turn it over and feel the bearings for smoothness. If you start killing glow plugs for no reason, it's usually a bearing going south. The other general problem is a loose head or back plate, bad o-ring on the main needle or air leak at the carburetor base.