As humidity is caused by moisture in the air its obviously going to depend on where you are in the world and the time of the year. Having said that the global warming thing seems to be changing the picture rapidly.
Here is a link to an interesting article on the Uk changes
http://www.rhs.org.uk/news/climate_change/chapter2.pdf
This map is also good for getting 24hr forcasts
weather.co.uk - UK - Relative Humidity Fcst (24 hr)
Normal room humidity is in the range of 40 to 70Rh so i think i would aim for >70 to 90Rh in summer noting that instrument accuracy may well only be within 5% anyway.
Some general recommendations here
Floriculture: Fact Sheets: Greenhouse Management