Lease up compensation as 3rd Party PM?

[Casey Kleinhenz]

Operate a property management company and primarily self manage. A contact has asked for a proposal on property managing his new  community (~60 units, high-end senior community). It is new construction, so active marketing would be a big part of the deal. He wants a proposal where fee isn't triggered until a unit occupies. 

I see a uncertainty in the lease-up period and uncertainty in the PM contract term and am struggling to come up with a proposal that makes sense for both of us. How would you structure PM compensation?