Non-contact Optical Turbidity Sensor

Dear people,

 

I'm trying to calibrate the turbidity sensor to detect bacteria E. coli in a liquid culture. However, I find this diagram a bit confusing:

 

1. Could you guide me to the specific formula for this curve so that I can write a calibration function in Arduino? Currently, the coordinates are a bit blurry and hard to see, and I'm struggling to fit the curve with a formula accurately.2. Is there a way to convert the turbidity values to OD(Optical density) values? Many scientific experiments use OD rather than turbidity.