Correct setting for koso speedo using magnetic sensor

Can anyone tell me what the correct settings is when using the sip koso speedometer with a  magnetic sensor. I have selected external , input tyre size and have option to input in the area that would be for  the gear ratio if using a normal speedo cable. It has two digits. First reads 0, 1 or 2. The second reads from 0 to 9. The third cannot be changed and is the letter P. Scooter is a lambretta gp but using gilera runner 180 on 12 inch wheels