You're never going to get folk to agree on how the mat should be cut, because it's subjective. The tradition of leaving a "runway" space at the bottom (I usually add 1/4 to 1/2 in) is just that, a tradition. I agree with Vaughn - a quick and dirty cut out of paper to the size you're thinking of using will tell you more than anyone here can about what you like. Personally, I simply avoid two things: a side border that is larger than the top border and a smaller bottom border. But then, that's just my tradition...