When you are measuring the square meters of the job remember to allow for the 200mm past the surface of the substrate for the thickness of the stone. Therefore if you are measuring a wall which returns around a corner, the stone wall will be 400mm longer than the actual substrate surface due a larger perimeter.
When measuring for sills measure the linear meters (length) required, don’t forget to allow for overhang on corners. Then add 10% for a margin of error. Sill stone is cut to a standard width of 250mm unless otherwise specified so that there is 50mm of overhang to the sill.