20% is the maximum legal limit for honey that is sold. The higher the water content the more likely it is to ferment, so the lower the better.
My concern is the variation in your readings, as just extracting would not lower the water content. Perhaps you just sampled some random cells prior to the extraction and were unlucky choosing some with a high water content.
Once honey has been extracted and settled I make sure I stir it well before taking a reading, as that can make a big difference.
What percentage of comb was capped?
As to time to spin for. No direct answer. Depends on viscosity of honey . As you say extract enough so frames feel light and honey stops spattering on side of extractor