For the 19" and 20" lcds the 900 pixel height is generally the native res while the 1024 does more while the 1080 tends to push things off of range as far as what is in view. The backdrop is first seen to in order to meet the native res requirements where you then resize the wide papers you make while still keeping the one on one aspect ratio intact.
The actual combined native res here for both the 19" primary and 20" secondary would be 1440x900 adding 1600x900 secondary to end up with 3040x900 while the 1024 replaces the 900 the slight increase to 3120pixels on the width becomes the winning formula. For 22", 24" displays you then recalculate things with the increase seen for those. Likewise a recalc is needed for larger or for the smaller screens.
Couldn't help coming out with that one! This is one of those papers you share with those who complain about the 10 app and not wanting 10 to come in and replace things on MS's terms.
All some really nice and creative papers guys