Yes indeed interesting. The usual comfort level that I was taught is 40% Relative Humidity on 21 celsius (which is as good as 70F in very old money). The old way to do this was to put 3 medium sized ice cubes on their own in a glass and leave it 5 minutes. Ideally there is a little condensation/ moisture on the outside of the glass. If it's bone dry then it's probably below 30%. If it's dripping wet on the outside then the air is already over 50% wet relatively speaking. My Manila party trick that.