Like Mickey said, have a look through the manual first up. The instruction help in Logix 500 isn't too bad by its self..
Bear in mind the PID instruction isn't a magic bullet for process control... Any loop where the PV responds quickly is usually easily tuned, but add delays from CV movement to PV response, and changing process conditions on top, tuning starts to get more complicated.
If your tank is big volume for small mm change, you are more than likely going to end up with tuning issues, especially from start-up.. If its a little tank (only a few minutes pumped full to empty) shouldn't be as much of a problem.
If your boss is looking for the pump to continue running rather than start / stop, the proportional band approach will still be easiest to achieve... Have seen this done without the PLC in the middle, straight level 4-20 into the VSD direct, and the analogue in setup to suit...
Cheers Chris