I wouldn't leave it more than 1" off. I've had a couple donuts and at less than 1" off level, the pool moves to the low side, stretching over time. It's very difficult (almost impossible) to build up and compact enough to support the weight of the pool. I recommend you dig the high side until you have enough level (measured level not looks pretty level), undisturbed soil under the pool. A water level is a good, cheap way to measure level across a distance. Ben posted this yesterday:
Quote Originally Posted by PoolDoc View Post
Take a look at this -- over long distances, a "water level" is more accurate than an expensive transit level!

www.youtube.com/watch?v=pusDOh_5kTc
A roto-tiller can make digging easier.