Mushroom Environment Control - Arduino Powered

Step 12 C02 Calibration

This one is difficult.
The way I went about this was to look at the current value of C02 ppm in the world today (yes it is going up!), it's around 400ppm. Assuming that it is a bit higher if you live in the city (my one is relatively clean) I assumed normal outside Co2 levels to be 500 ppm.

So I opened my window and after allowing the Co2 sensor to heat up I took a number of 
Serial.println(carbonD[4]); 
readings. 

I averaged these out, and set my X1 value to equal this, my Y1 = 500.

The datasheet showed that the sensor maxed out at 10000ppm, so for my second reading I got some bicarb and vinegar put that in a plastic bag with the sensor and sealed it up and set the reaction off.
After a minute or so It reached its lowest reading using
Serial.println(carbonD[4]);
so I used this value as my X2, my Y2 I set to 10000.

Now I solved to find M and C: (or use excell :)  )

y1 = MLogn(x1) + C
y2 = MLogn(x2) + C

And plugged these values back into the program:

#define Carbon_M -83.45 
#define Carbon_C 768.62 


By the way for this step I am not convinced about my assumptions made about X2 and Y2 values (or Y1 values for that matter :)  ), if anyone has some suggestions I am open to them.