Hi Martin,
I recommend asking your friendly FS-PM functional lead on the project 
FS-PM has a functionality for simulating the commission case in ICM. In FS-PM click on "Commission" in the toolbar, then on "Remuneration Result" in section "Commission Simulation" to start the simulation.
Currently, this functionality is only available on the Dynpro screen, there is no service/BAPI to call from the portal.
Cheers,
Michael