Yep... another way of thinking about it is to take the yard of mulch and knowing how many inches you wish to add, figure out how many square feet each yard will provide. 27 cubic feet are in a cubic yard. That means 27 square feet 1 foot high. Because you require less mulch as the depth decreases, you can multiple 27 by the reciprocal of the fraction of a foot... EG at 3 inches multiple by 4 because 3/12's reciprocal is 12/3=4.
1 cubic yard gives you:
27*(12/4)= 81 square feet at 4 inches high
27*(12/3)= 108 square feet at 3 inches high
27*(12/2)= 162 square feet at 2 inches high
This way you also know how many leftover sq. feet you will have as well to plan accordingly.