Conversion Ohm to PPM

Hi. I want to convert my readings in Ohm to PPM. Specially I am aiming for CO, NO2, NH3, H2, CH4

In MiCS data sheet i saw that RED sensor can read that? On forum I have found conversion code like this:

red_in_ppm = math.pow(10,-1.25math.log10(readings.reducing/red_r0)+0.64)
ox_in_ppm = math.pow(10,math.log10(readings.oxidising/ox_r0)-0.8129)
nh3_in_ppm = math.pow(10,-1.8
math.log10(readings.nh3/nh3_r0)-0.163)

is it okey? maybe someone have better one and for H2 and CH4