I think I made a fundimental error right at the start. I calculated mV per count incorrectly.
Instead of 4.9-0.2/1024, I should have done this...

4.9/1024=0.00478515625v per count. Reason being, the ADC does not start at 0.2v, it starts at ground.

The sensor Im using is a Motorola MPX4250A. After re-reading the data sheet, there is a chart on there that shows an output of 1.8v at 14.7psi - which is what Im getting.

So I should be able to work out what 14.7psi is in counts...

1.8v/0.00478515625v =376.163265. or 376 in the real world.

Therefore counts per psi = 376/14.7= 25.578231 rounded to 26.

So, now I know that anything below a count of 376 needs to have a display of zero, how do I create the offset? Or, better still anything below a count of 376 needs to be in a different scale for measuring vacuum. And, have do I make 26 counts equal 1 psi?

Thanks for sticking with me through this.

James