In this example, we are going to cover a floor area with a pattern of tiles of two sizes. 

The template contains 150 mm x 150 mm tiles and 150 mm x 300 mm tiles. 
 
The proportion of small tiles is 30% and 70% for the large tiles. 
 

First, let's measure the area to be covered with the Area tool : 

We are going to associate the two sizes of tiles and add a 15% waste factor: 

Let's start by the small tiles (30% of the area) :

The [Net Area] expression returns the area measured on the plan. The "0.3" constant is for the 30% ratio and the "1.15" constant is for the 15% waste factor.

Repeat the operation for the large tiles (70% of the area) 

The [Net Area] expression returns the area measured on the plan. The "0.7" constant is for the 70% ratio and the "1.15" constant is for the 15% waste factor.

Result : 142 and 165 tiles.

We could now add some grout, glue, labor etc. to this area by using all the power of the formulas.