It sounds like you are trying to emulate a typical valve postioiner in your PLC. No problem.
First, you need to establish a bandwidth setting - this is simply the error you will allow between the desired percent open and the actual position feedback before you move the valve. You add and subtract half the bandwidth from the nominal position target, and this gives you two values - a minimum threshold and a maximum threshold.
Then you need to convert the position feedback data to % open. This is a standard scaling problem, and in the ML-1200 you can use the SCP function.
Then you need two compares. If the feedback is greater than the max threshold you turn on the output for closing the valve. If the feedback is less than the min threshold you turn on the output for opening the valve. If the feedback is equal to or between the min and max threshold you do nothing.