To answer the OP and reconfirm it will decrease your highest level craft first. Then at 75, 75, and you skillup the craft at 70 it will alternate which craft it lowers, 1 .1 from one then .1 from the other... repeating.
This is not correct. In my experience (I have seen my crafts decrease by .1 over 1600 times) there is no rule as to from where the game will take that .1 skill. It seemed to favor the highest craft, but by no means did it delevel my highest craft every time.