MG811 sensor problem

Hello, this is the first time I'm dealing with arduino and i need some help with my project. I'm trying to measure CO2 concentrations inside a greenhouse using MG811 sensor on arduino uno. The power supply is 6V. I changed the DC_GAIN in the sample code (https://sandboxelectronics.com/?p=147#Sample_Code_for_Arduino) from 8.5 to 6. I guess I should change ZERO & REACTION VOLTAGE too but i don't know the right values. I'm using only analog input and therefore I deleted everything that refers to BOOL_PIN. Although I'm facing problem with the measurements which are absolutely wrong. I believe that the problem lies on zero and reaction voltage. Any help would be appreciated!

Post the code you're using and a wiring diagram of your setup (as you seem to have changed both from the version provided on the linked page).

How did you do the calibration?