Are the pressure sensors reading from the same reference point (see the post earlier which talked about relative vs absolute).
Have you tried to print intermediate values of the calc to verify it is what you expect?
It should not matter to the arduino, but have you got a stable 5 v voltage? Arduino will return a value referenced thru its own voltage, but if the sensor expects exactly 5 v you may have an issuers it is not accurate.
Again, post your code. It seems to me that you could be getting issues due to the order of you (integer) math operations. What is the percentage error that you get and does that increase with higher pressure? Is the other pressure señor a more accurate sensor? How do you know? The sensor you have has a 2 % error, so the absolute error will increase with the higher pressure.