I think the most common way to determine the % is weight/weight or volume/volume when both are liquids. Weight/volume is not so common i think but it is true that it is used too
Since the production of DHT is an enzimatic reaction and enzimatic reactions are in many cases temperature dependent I guess that, to some extent there could be a relation. I would be a cheap and interesting experiment to measure the scalp DHT before and after exposing the skin to low...