A diagonal layout looks best (IHHO), if the outside consists of half-size tiles up against the boarder, with no small pieces. It very much would depend on the size of the room. I'd consider, if it was close to achieving that, to adjust the boarder slightly smaller or larger to accomplish that. You'll get a lot more opinions on this at www.johnbridge.com. The first thing, though, is to verify that the floor structure is strong enough for a natural stone tile installation. The average home does not have a wooden subfloor strong enough to install stone without some reinforcement. The subfloor requires two layers of plywood as well. Check out that site...you may need to reconsider. Cutting the stone on the diagonal, accurately, is also a little tricky and requires a larger than normal saw - on the diagonal, you need a saw that can cut nearly 17".