this is an interesting topic, I've done my own mat board mounts too, and so far the only "rules" I could find were to go to the next frame size up, which seems to work, and also regarding making the bottom margin larger than the bottom. Most of mine looked ok like that, but i've never been too happy with them.. I also couldn't decide which colour went best with the photo, but i'm guessing this is another thing learnt over time?

I've seen a lot of the fine art prints that are square, and usually have a massive border around them, with something like 3 to 4 inches border at least all round for a 4 inch print etc, and it does tend to make a lot of difference, having a big frame around a photo does seem to make it more striking.