At Interstone Marble and Granite, we like to think that the rule of thumb when buying materials is 15% – 20% extra for wastage. If you have a bit left over from the installation, it’s a good idea to hang onto it just in case something should happen. Natural stone can vary in color, therefore, if you need more, your material may be discontinued or from a different batch thus not matching with your existing stone. Have fun with your project! I’m sure you’re going to love it.