The relationship between the motor frequency and speed is linear. Zero Hertz corresponds to zero RPM. 60 Hertz corresponds to nameplate speed of the motor, assuming the VFD is configured correctly.
The following is probably going to sound harsh to some people. There will be those who say that everybody has to start somewhere, that there is no such thing as a dumb question, yada, yada, yada... I'm going to say it anyway.
Although disturbing, it isn't all that unusual for the people programming PLCs to lack the background in physics or mechanics to be able to do simple calculations such as you requested in your initial post. Sometimes it's because the people doing the programming have a background in electricity or electronics or computers rather than in mechanical systems.
Unfortunately, you also seem to lack a basic understanding of how VFDs control AC motor speed. This leads me to wonder what qualifications you bring to the task you've been assigned.
I strongly suggest that you spend some time learning about the devices your PLC will be controlling before you write any more PLC programs.
