Vertical Page Alignment

Center - pages will be vertically centered (horizontally, if you are using vertical spreads)

Custom - entered value means how far will the page be shifted from the spread center. The value is a distance between spread center and top of the page. Negative values are above the axis, positive bellow the axis. If you have two letter size pages next to each other, the offset value to have them centered is -5.5 inches (you want the top of the page to be half the height of the page above the center).