we always reccomend 10%.-15% heringbone if set diagonal will render many of the drop cuts useless (more so than on a square layout.). 15% -20% overage and choose a quality installer. A novice may require more.

Another aspect of this is the type of tile you are working with. Since you are working with a Porcelain tile as opposed to a natural stone you could probably get away with a little less overage because you'll probably have fewer cracked or chipped pieces. 15%-20% as indicated above would leave you feeling comfortable on completing your project without any issues.

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