Absolute and Relative sizes to set area/content height

Hey all,
So i have a document that is 8.5in by 11in...
and i want to have a static header, lets say 1.35in in height...

First question, would I set that header area's extent to 1.35 inches and not
in pixels right (not sure what happens when rendered/viewed on other
machines).?

(to answer next question...i just have an empty fo:block-container in that
header area, that has a background image... I set it's height also to
1.35in)

Second question, I figured to get it looking best, the header
background-image is about 100px in height (75 PixelsPerInch resolution)  ...
if rendered/viewed on other machines, will it look different?

Should i set fonts in pt's or pixels. what would look more consistant when
re-rendering/viewing on different machines. (might need a refresher on
absolute and relative sizes)

Thanks,
Ari