Unless it's a really skinny mat, say an inch and a half or less,
I usually add at least 1/4 inch to the bottom of the mat.
There are some general guidelines that we learn, and that become
so ingrained over time that we start to think of them as intuition.
These would be things such as, it's good if the amount of mat
visible in the finished job is not equal to the width of the frame
as seen from the front. It's also important to find a good visual balance
between mat width and frame width. Often, a narrow frame looks good
with a fairly wide mat. To skinny of a mat with a narrow frame and
the mat and frame start to look like boxy little lines surrounding the art.
The wider the frame, it seems like one can go one of two ways. You can
make the mat very narrow, to look like a liner, as when the frame is
three inches wide and the mat is 1 1/4. Or you can make the mat nice
and wide. The wider the frame, the more mat it takes to look like it's a wide mat.
The size of the subject matter in the art is an important factor in choosing mat and frame width, too. A piece that has large, bold shapes and/or colors can often handle a wider mat and frame. The poppy paintings by Georgia O'Keefe would be a good example of this. If, on the other hand, the subject is tiny and delicate, a wide frame can
overpower and dwarf the art. A soft watercolor that has a tiny line of cows on the horizon might do better with a frame that's an inch and a half to two inches wide, rather than three. As with most guidelines, there are certainly exceptions to this. I've seen plenty of times when someone would frame a tiny painting with a very wide moulding. The way they pulled it off was to have a really wide mat, so it seemed like a vast space around the art and removed the frame from becoming a visual competitor with the art. I'm thinking here of something like a three by four inch painting matted with a mat that's five inches or more. This kind of exaggerated space often also features a mat that has a much wider bottom than top and size. If, say, the top and sides were six inches, the bottom might be eight. I've hardly ever done this, but
it can look good.
The few times I've done unconventional mat widths, they've turned out
nicely, but I mostly do top and sides equal with the bottom slightly weighted.
Once, I framed a little hand-pulled print of an owl with an unusual mat design.
The print was about 3x4", and the owl was on a branch looking down and to the it's right. I cut a mat that had the print up in one corner, with not much room above it's head and behind it's back, and a really wide mat in front of it's 'view' and underneath it. The result looked really good. The top and one side were about 1 3/4 and the other side and bottom were about six inches.
The only other time I've really played with perspective to an extreme was
for these tiny photo strips a college photography prof wanted to frame.
Each one is about 1 1/8" high and 5" wide. At his encouragement, we chose
really wide designs, and they looked great. The top and sides were about
two inches and the bottom was six. It made them look like something
interesting, rather than the way they would have disappeared if
just put into tiny mats.
I know this is a long post, but if any little bit of it sparks a good thought
for you, it will have been worth all the tap-tapping.
Rick