In this example, we are going to calculate the area of a sloping wall with the Length tool. 

The formula will generate 4" x 4" tiles to cover this area on one side of the wall.
 
The wall has two lengths of 4' 3-15/16", here represented by the [Length] variable and a height of 4'0" on one side and 8'0" on the opposite side.
 
The formula will be written like this:
The ((8'0" + 4'0" ) / 2 ) expression represents the average height of the wall (8'0 on one side and 4'0" on the opposite side).

Result: 468 tiles.