In order to produce steam, how full should glass be? Half? Less than half?

As long as you can see water in the glass it is OK. Depending on how big the system is the level will could vary. When the system is first started it will be at its fullest. As the water boils and goes out in to the system the level could drop. Does the system have an auto fill or do you have to fill it manually?
The system will not work if the system is overfilled, meaning the boiler were totally full of water. If the sight glass is full of water to the point that you can't see the level then you need to drain it down until you see the water level.
Hope this helps

I find that if its less than half, the steam is "weaker" and slower to get into the radiators. Is this normal?