I realise that this is an old post now, but, just got one of these to make my own weather station.

In regards to your readings being out by a factor of 10, the datasheet states that the formula is for calculating kPa, not hPa, so that's why it's out by a factor of ten.

The formula can be simplified a little for the Arduino too.
As the 'Vout/Vs' part is just a ratio, you don't need to calculate the actual voltage. This saves a few steps in the calculation and will reduce the error too.

Also, as you might not be supplying exactly 5v to the sensor, working the whole thing out as a voltage probably won't be totally accurate, where as 'analogRead/1024' will always give you the right ratio.

So, to work out the pressure in hPa, or millibars as we call them in the UK, and get the most accurate reading use:

P= ((analogRead(0)/1024)+ 0.095)/0.0009

Regards,
The Cageybee

*(Updated the formula. Took away a zero from the final division when I should have added one)*