Commercial hops are dried to about 8 - 10% moisture content. I dry mine to between 10 and 15%, since I don't want to over dry and lose too much hop oil content. I built a hop drier, that heats to 125 F, and get the required dryness in 10 - 12 hours.
I measure moisture content by weighing 10 grams of the dried hops, putting them in a 200 F oven for 30 minutes, cooling and reweighing. The percent moisture is calculated this way:
original wet weight - dry weight = Moisture content.
% moisture = moisture content/dry weight X 100
For example, 10 g wet hops dries to 9 grams = 1 gram moisture
1 gram x 100/ 9 grams dry hops = 11% moisture.
I was surprised how much moisture was retained in hops that looked and felt dry, but had as much as 35% moisture left in them. Drying to a consistent moisture content makes bittering calculations easier, from lot to lot.