Hey Rennie, just carefully rip yourself a very straight 6' board (78" would be even better) and tape your 4' level to it. Voila'....6' foot level. Double sided tape works good too.
Using a longer level allows you to not only get the hinges plumb so that the door hangs properly, it also allows you to get the jamb straight using your shims, giving you an even reveal between the door and jamb.
My diet is working good. I'm down to needing just one chair now.
"Just think how stupid the average person is, and then realize that half of them are even stupider!" --George Carlin