How to pick right frame with mount size for a specific sized photo?

[UberTiger]

Not sure if this is more of a home question or photo question, and not done this in ages, so just wanted to check.

I have a a photo that is 17cm x 22cm and I'm trying to find a frame.

If I take this 4x6" (15 x 20cm) one for example, would that be the right size?

The thinking being the stock gap of the paper mount within the frame is 15 x 20cm, but the full product size is 20.7cm x 25.7cm.

The photo sizes given are with the card window mount in place. Remove the mount and you can display a slightly large picture. The inch measurements are rounded measurements worked back from standard centimetre sizes.

That means I could take a Stanley knife to the mount and increase the size around the photo to the 17 x 22cm I need.

Is that correct?