Due to the normal manufacturing tolerances of +/-2% on all dimensions, we strongly recommend that you do not cut any worktops or mounting surfaces until you are in receipt of your sink. It is best to use the sink as a template to provide accurate dimensions.
Spend time identifying a specialist professional you feel comfortable with. Online review sites can be helpful in forming a shortlist. Judge their understanding of the project just as much as the quote. It may be worth asking to see one of their previous jobs, or talking to a recent customer.