One thing that springs to mind is that you can't tell what pressure is present before you start. Once you press that button you are committed!
Imagine that it has been a day or so after some one else has set the pressure, then you come to set it.... What was the initial reading.
I would turn the problem on its head a bit.
Make a measurement system that continously displays the reading from the device then add some logic to handle the relative pressure.
The continous pressure measurement can then be fed to graphs for trend analysis such as leak detection or data logged etc.... You then always have a reading of the absolute pressure and can store the offset value. Thus you can also correct for calibration problems should you desire and also monitor the set differential points as well if required.
You get better feedback for the operator and it would also work for calibration, maintenance and debugging.
