[SML] Ratio of exit doors to audience?

[mouse at fieldmousepro.com]

I seem to remember reading about the calculations for determining how 
many useable emergency exit doors are required for a space, based upon 
how many people will be in the room. The situation I'm envisioning would 
be for a corporate gig in a hotel/convention center ballroom where some 
of the doors around the perimeter (backstage, for example) would be 
blocked by scenery or pipe & drape for an event, but the room would not 
be fully populated with seats, either.

Unfortunately my brain and Google-fu are both failing me at the moment. 
Does anyone know the rule-of-thumb for this?

Stephen