dont stagger your wheels size too much... it'll make for shitty understeer when pushing the car hard. Besides, not putting the widest tire you can on the rear makes the sidewall more stable and responsive

I'd probably way something like 225/45 18 front and 255/40 18 rear... that way you'll also have the same rolling diameter (ok, so its off by like 2/10ths of an inch..... front circumference would be 81.6 in and rear would be 81.8)

have fun with this....
http://www.miata.net/garage/tirecalc.html