Pi Settings On G.e. Vfd Model No. 6kxc143003x9a1

I have to to keep at the maximum effciency the running of a 250 HP mill motor that grinds animal pellets. This pellets are fed by a rotary valve that has a 1.5 HP motor controlled by the referred drive. I have a a KW transducer , EMOTRON M20 , which senses the power demanded by the 250 HP motor and its analog inverter output 20 - 4 mA is fed to the analog input of the drive in such way that when the load is low the VFD should speed up the rotary valve ( feeder ) in order to increase the load on the mill motor. this should be done automatic through the PI controller on the drive. My question is which values should I place in the PI controller at the beginning and how do I know that I have reached the optimal values ?. please think that I do not have any experience at all with PI controllers.
I would be very grateful for any help .

comapro