Analog pH Meter Kit CALIBRATION

Hi Everyone, 

I used this product: https://www.dfrobot.com/index.php?route= ... 1B5GpGLSUk

so I was wondering how to calibrate my ph reading? 

I have two calibration liquids. PH 7 and PH 4. 
The sample code gives me slightly wrong readings. 

How do I calibrate the code, so that my reading correspond to the ph of the liquids I use ?

I used this with my arduino mega. 

Regards