I don't want to spoil your day but i must inform you that you probably will need to spend some more $$$$$ on a new sensor. If you always want a zero reading with zero pressure applied, regardless of the atmosperic pressure, you should have bought a gauge(or differential with one side open to surrounding) sensor. That type of sensor measures pressure relative to the surrounding(atmosperic) pressure. Now you have an absolute sensor that measures pressure relative to an internal vacuum, a common use for this type of sensor is to measure atmosperic pressure.

Or perhaps i haven't understood what you're trying to accomplish, what exactly is it your trying to build?

/Ingvar