Dear Benoit Lefebvre
Yes,Yes, you aright. Change the fraction in the OPT-PROD cards not the OPR-PROD, I misspoke.
As for how it was discovered that the photon count remains the same after altering the fraction, you can refer to the question post in the link below, which was also posted by me. I used the USDRAW entry in mgdraw to record the light emission of each muon entering the scintillator. After changing the fraction, I compared the results and found no variation in the light emission when muons entered the scintillator. I am quite puzzled by this.