Humidity Levels required to prevent static discharge?

We have a Butvar spray room. The room contains toluene and Prestoline. What is the required humidity level to prevent a static discharge. We run the room around 40% humidity. I have read that the safe level is around 50%, is this number accurate?

I work with ordanance systems for a living and we reqularly check our temperature humidigraph to assure we never dip below 35%. If we do, we cease operations due to static discharge potential.
Another tool we use is called a static meter. It actually measures the static potential of charged objects and fits in your hand. When voltage levels exceed predetermined levels, we stop work too.
I also used to work in clean room areas with high VOCs but they were MEK / methyl ethyl ketone fumes and I offset them by upping the humidity to 55% and temperature to 70F. I experienced no static issues at over 4000CFM of airflow in the room.
Good luck!