When you deal with surface or volume make sure you get right dimensions at the end. If you multiply one surface area by another surface are you will not get a surface area, because in this case you would get:
That doesn’t make any sense and should ring a bell that calculations are incorrect.
On the other hand, when you calculate a surface area of a complicated figure you can break it into simpler ones and then sum up the figures. In this case the dimensions are Ok: