I'm not a fan of running shorter sidewall in the rear - but maybe you like that? I know some people do. 245/45-17 is 25.7", to keep the same proportion w/ a 18" wheel you would want 27.2" in height. To keep the same sidewall you would want 26.7". A 275/40-18 would give you the same sidewall height. If you can test fit a 275/40-18 and see how it looks. My opinion (and that is all it is) would be that a 275/40-18 would have a better look. You also would get the extra cushion of the taller sidewall.
My front tire is a 255/40-18 (26" tall). My rear tire is a 295/40-18 (27.2" tall). Unfortunately, since the wheels are both 18", this gives the visual effect that my rear wheel and tire are smaller. Better would have been 345/30-19 which is also 27.2" tall, or 295/35-18 which is 26.1" tall. Unfortunately the tire I have is not available in either of those sizes and the 19x13 wheel I would need for the rear is not so cheap either. For now I will "suffer" with what I have. Once the car is on the road I can look at other options.
One that I am leaning towards is 275/35-18 front and 325/30-19 rear. 25.6" front, 26.7" rear.