I have started to use a cadence sensor. I read that an ideal reference cadence is around 90rpm. I've been looking at my cadence and found that I can easily ride at 90rpm (and higher) when descending with little effort. On the flats, at normal speeds, I seem to pedal more around 80rpm, only reaching 90rpm when I want to push myself and go faster than usual.

However, when climbing any meaningful slope (10%, for example) I found I'm totally unable to pedal at 90rpm. I feel really uncomfortable at that cadence, and end doing most of the climbs between 70 and 80, and, sometimes, even falling down to 65rpm if it's a really steep slope (16 - 18%).

What is a usual cadence for such climbs?