Issue reading pressure transducers and getting floating value
The analogRead() method returns an integer between 0 and 1023 on a 0-5V swing.  Your transducer produces a 0.5V - 4.5V range (from the datasheet you linked to).  That's a 4V swing. So analogRead() can be expected to yield values covering from about 102 to about 920. (10% or 0.5V off each end).

The pressure range of that transducer is 0 to 300 psi (again from the datasheet). So I'm wondering if 500 psi is a good test, you didn't blow the device properly, but that's certainly outside its range of linearity.  If I plug 102 and 920 into your formula, I get ~0 and ~300.  So your formula is good.  I cannot explain why it's unchanging, other than perhaps you damaged the device with pressure at least 66% over its rated pressure.

Please also see this from the documentation:


  Pullup resistors
  The analog pins also have pullup resistors, which work identically to pullup resistors on the digital pins. They are enabled by issuing a command such as
  digitalWrite(A0, INPUT_PULLUP);  // set pullup on analog pin 0 
  Be aware however that turning on a pullup will affect the values reported by analogRead().
  Details and Caveats
  The analogRead command will not work correctly if a pin has been previously set to an output, so if this is the case, set it back to an input before using analogRead. Similarly if the pin has been set to HIGH as an output, the pullup resistor will be set, when switched back to an input.


So you may want to remove the INPUT_PULLUP verb if you used it earlier. 

EDIT

I think you might have it connected wrong -- following the above formula, ~330 is about what you'd get if the digital pin got ~5.0V.  So may want to check your connections.  With some pressure on it, a pigtail on the ADC pin should show &lt;= 4.5V.  If not then you have something connected wrong.