How steep is the hill? If it isn't that steep, and you're going up it at over maybe 16-17 mph, 80-90% isn't really pushing that hard. If it's steep enough that you're doing 10 mph, 80-90% of your personal best is probably good.
Why? Because if you're going fast enough that wind resistance plays a big part in determining how fast you go, going only 80% as fast means your power could be down by almost as much as 50%.
Assuming, of course, you just about killed yourself to get that personal best.