The Configuration Options tab of the Refine Resources/Costs/Skills dialog contains a field where you can specify a maximum percentage of fixed personnel for each time period.  If you enter a “50”, SLIM-Estimate automatically scales back the fixed resources to comply with specified percentage.  In this case, the total number of fixed personnel in a given month will never exceed 50%.  For each time period, the max resource effort hours are subtracted from the total effort for that time period, then percentage-based skills allocation settings are applied to the remaining monthly effort.