How to calculate cost of managing common areas

[Vern McDade]

Hey all,

So I successfully manage 3 units in a 6 unit Multifamily building. The owner has now asked me to manage the common areas. Ability and experience is not the issue. What is, is a way to best define the management fee of just the common area. There will be contract services (trash, pest, cleaning, etc), PMs, and the admin portion of bill pay, sprinkler, fire and backflow inspections.  A flat fee might do, but is there a standard in common area only management fees in 3 story brownstones?

Suggestions welcomed.