Heatsink surface temperature
Clarification 1

The re-arranged equation to determine the required thermal impedance of a heat sink is

\$ R_{\Theta s-a} = \frac{T_j -T_a}{P_d} - R_{\Theta j-c}  - R_{\Theta c-s} \$ 

So assuming you have used the correct values then yes, this essentially sets the maximum Heatsink ambient the system can work at.

Clarification 2

And this is a real problem. One solution is a valid Finite Element Analysis of the thermal capability to determine how efficient a certain fin topology is at removal of heat to the surrounding air

another solution is to pre-empt the optimum fin consideration: If the fins are too close the heat transfer coefficient decreases, if the fins are too far apart the heat transfer coefficient equally decreases due to a deduction in surface area... 

There are some equations that help determine the optimum fin configuration.

Then there is empirically. The usual rule of thumb when passively cooling electronics is the heatsink will be at a temp range of 35-45C (w.r.t. a 25C room ambient) if the heatsink is exposed to the room. If the heatsink is enclosed or downstream of another thermal producing device then a local ambient of 50-60 is a reasonable 1st pass starting point