How big do you make your mattes?
I realise that it's largely a matter of taste, that some images would demand larger or smaller mattes than others, of course, but are there any "rules" to guide a beginner?
I had a great lesson about custom framing from a private photography dealer who did a lot of sales to and framing for an interesting collection of national and international galleries.
What he would do himself was to lay a nice clean sheet (not white -- mid gray works well) on the floor, place the print in question on the floor, then stand on a chair and look down at it. He'd get a stack of 8.5 x 11 paper from a copier and fold the papers down to create a boarder size and go all the way around the print. Climb up on the chair and look at it. Come down and make changes. Iterate until done.
This method works. Better than you'd believe from my poor description. The advantage is that you work with the real print at the real size. It not being theoretical really does help somehow.
What I've found doing this myself is that most of my own prints want to have a little more boarder on the bottom. So the top and sides are the same width, but the bottom is a little more. Usually it's not much more. I've got a few with four and five inch boarders where the bottom is just a half inch bigger. But when framed, that extra half inch can make or break the presentation.
I would not have believed it coming from anyone else. But having seen some of this guy's impeccable presentations I had to take him very seriously. He put the "custom" in custom framing. And for a very good reason.
He was also fairly emphatic that the frames be hung fully leveled and that all prints (for photographic exhibitions like a one person show) on a given wall be framed the same way (frames and matte style, but not necessarily the same size) and have the top of their frames all line up perfectly. When done correctly you don't notice any of it, and all you remember are the images. Which is the point of the whole exercise IMHO.
I've done this in my own house. Sold a few prints. And no one ever talks about or asks about the framing. Make of this what you will.