In the facility income rules you will find that your income is dependent on the Planetary Income Modifier or PIM in short. This PIM value is the only thing that is influenced by the employment rate (ER). The PIM is a function of ER, CR and TL.
If you take the CR and TL to be constants, you can solve the equation to figure out for what value of ER your PIM is maximal. This is the 1.5 value that Mikel mentioned.
If you take the TL to be constant, you can make a 3D graph to give you the dependence of the PIM on the ER and CR. This shows you how the PIM is more sensitive to CR than to ER. (in CR direction its linear, in ER direction its parabolic)