The spec that Nick has provided is the one that I've been accustomed to using, although I haven't done much resi work for years.
When we have to flush mount anything larger than a quad recp or double switch, it must be "boxed out" with drywall so that it actually lies in a recess that is outside the firewall's envelope.(usually a big pain). When possible,(unless it will project objectionably into a fire corridor or the like) we surface mount panels and such, and for aesthetic purposes will furr out and drywall around them.
