Refrigeration compressor Vi control

I would like to control Gram compressor by S7-314
I know how to control capacity increase and decrease.
But question is, What's condition I should adjust Vi ratio?

The Process Variable (the parameter that is being controlled) would be the temperature inside the refrigerator. Normally, most refrigeration systems use ON/OFF control where the compressor is either running or not, but a larger refrigeration system could also be controlled by compressor speed, by number of compressors running, or by a variable compressor unloader valve.

You could use the suction (LP) pressure.