Well just for reference, I obtained some numbers from http://www.1010tires.com/tiresizecalculator.asp
This is more for an accurate rolling diameter and not for anything else:
So if you were going for the most ideal tire size on 20 inch wheels, it would be 235/35R20. Though, you could even go for 245/35R20. It may then read on the speedo 1.67% slower, but with the amount that the speedo is already off in the Aurion, this wouldn't be an issue. Plus as well you can get the Potenza Adrenalins in that size, lol.
If you do want the 30 profile though for looks, you should go for the 245/30R20. You load rated tire choices are limited, but you would probably be looking at something like a Maxxis MA-Z1.