We would always recommend creating a plywood or cardboard template before starting to cut your Slate hearth blank. You need to fully measure all sides of the opening and establish what overhangs you are going to need to meet current building regs, link to the guide below:
Indoor Fireplace Hearth Installation
When you have transferred these measurements onto the plywood, cut this out carefully and then test this template in the fireplace opening to ensure you have correctly measured. When this is correct the template can be used to mark out the Slate ready for installation.