The nominal compensator value is set by first running an optimization according to your criterion before beginning the tolerance analysis. For example, let’s say you were running a tolerance analysis for spot size and you had your back focal distance as a compensator. Before running the tolerance analysis, OpticStudio would build a default merit function for minimizing spot size and run an optimization to get the compensator in the ideal position. So, the nominal value in your tolerance report will reflect this newly optimized value.

If you do not want OpticStudio to run this pre-optimization, you will have to use a Tolerance Script with the keyword:

NOMINAL OPT_OFF

More information on tolerance scripts can be found here: How to get any optimization operand value in the tolerance report