See Montgomery's Design and Analysis of Experiments the response surface chapter. The book can be found in the z-library google instructions for it's use. D. Booth
Fabio Fornari : Thank you so much for the explanation.
I have two factors with three levels for my experiment.
I have used two factors-three levels in CCD-FCC with alpha = 1 according to this theory https://www.itl.nist.gov/div898/handbook/pri/section3/pri3361.htm . Do you think that would be okay to use?