You count the shortest leg. If there are 2 generations from the common ancestor on one side and 17 on the other, that's 1st cousins, 15 times removed. That is, one person is 15th in descent from a 1st cousin of the other.
If Diana is your 10th cousin, you're both at least 11 generations down from the common ancestor.
But with Henry, you're descended from his grandparent, and so from his 1st cousin.