Proper monitor volume?

[destriaero]

This might sound like a stupid question and I am not sure if the volume is up to preference.

But if not, what is the proper monitor volume for mixing? For example, I have Behringer MS40's (40 watt), so what percentage of volume should I be using for a truthful mix?

Thank you.

 

[Massive Master]

http://www.massivemastering.com/blog/html/blog_files/Calibrating_Your_Monitoring_Chain.html