Our mount cutter uses millimetres when cutting, some make sure your measurements are accurate.
Use the back of the frame, between rebates to measure the size. You can also measure the glass or the backing if they fit the existing frame with no gaps.
Allow a 5mm border all around your image, so it sits well under your mount.
We automatically add a bit extra on the bottom to all bespoke mounts. That gives perspective to the framed image when it's on your wall. It's not a noticeable difference but it makes the frame more proportional, we definitely recommend it.