In order to maintain my new "rep" as a vast holder of information here are some basic thoughts.
generally speaking the top and side are the same size with the bottom being at least 1/2 inch deeper if not at least one inch. I don't like centered windows.

Also, for most hangings the work is salon hung which means the frames are all hung vertically. As yours are square that should not pose any problems for you.

My own personal taste is to have deep boarders around the sides and a much deeper bottom. Lots of white space, setting up the image. As an example I have a 4 X5 print that is sitting in a 19 X 21 window. This may be too much for some, but i like it and people respond positively.

If these were mine I would have a 4 inches top and sides with a 8 inch bottom. Maybe more , maybe a little less, would have to think more about the specifics.

Something i like to do when matting a new size is to take a piece of white paper, cut to the outer dimension and a piece of colored paper the size of the print. Tape the color paper to the white piece and tack it up in the lab where I can look at it for a few days to determine if this is working for me.

As with everything else in our world there is always more than one way to do things.