Determination of spread-out Bragg peak (SOBP) inside media other than water is important for research or clinical purposes. Current study aims to characterize the optimal "p" values needed for the simulation of proton SOBP inside some dosimetry media using MCNPX Monte Carlo code. Following the provided data by ICRU-49 and applying the Bortfeld and Jette recommendations, the "p" values were determined for muscle, compact bone, and PMMA. Then, "p" values were optimized to reach accurate weight fractions for the Monte Carlo simulation of SOBP curves. Obtained optimal "p" values can produce accurate proton weight fractions for flat SOBP simulation. The slope of the SOBP region was highly dependent on the "p" value, so small changes in this parameter can largely tilt up or down the SOBP. The tabulated optimal "p" values can be reliably used for proton weight fraction determination during the Monte Carlo simulation of the proton beam SOBP curve inside the investigated media.