Hammer,
Good Job, Thanks !
Altitude does make a difference in some planes. As you climb, you can watch the GPH change dramatically on some planes, on others it hardly changes at all.
A good compromise would probably be to list the times at 5,000 ft. Although what you've done is probably perfectly fine. This is a good starters guide, it's up to the individual to learn the quirks of their favorite plane.
I noticed you still have the old field maps posted at NetAces. Are you going to use the field maps I sent you ? The only thing that is incorrect on them that I've noticed is the location of the map room at Vehicle Hangars has been moved from when I .pdfluffied the map package.