How high the AC Momma?
I suppose the measurement question has been figured out by now, but for other owners of HiLos... try this. Providing your HiLo is setting on a driveway, or level surface, place a step ladder next to it. One member climbs the ladder.
Place one end of a 2 by 4 at least 8 foot long, (or use a long rod, or pole), on the top of the AC unit. Family member on the ladder raises the opposite end until you think it looks near horizontal as possible. Measure the distance from the bottom of the 2 by 4 to the ground.
One thing to remember though. Most garage door openings have a slight rise from ground level into the garage. This keeps rain water from flowing into the garage. Just remember to make compensation for the fact a trailer will rise also as it goes into the garage also.
Lots of Luck...