A few weeks ago I came across a diameter calculator that looked a lot like the Miata one, http://www.miata.net/garage/tirecalc.html but it brought up a chart that showed different combinations for each rim size, and you could follow the green area to show any combination that was within a certain percentage from stock.

I cant find the thread where I found that for the life of me... anyone know the link? I'm sure it is right in front of my face and I'm just looking too hard.