Do you mean how to dentifying the model you are going to use? Or how to identify the optimal model equation? If I understand you clearly, you have applied Tanguchi optimization method via the RSM. What software are you using for you optimization process? If you provide some clarity on your question I can be able to help with useful informations.
A good optimization software to apply the Tanguchi is via the RSM using the Design Expert software Grace Rebekah. The Design Expert software is user-friendly and provides a platform for the application of the Tanguchi or any other methodology for the Design of the Experiment.