I run 657mm on a lowered Skoda Octavia that originally came with 205/55r16. They rub on hard cornering because I've got 7.5" ET45 (4mm more outward poke than your rims). 4mm doesn't sound like much but it does make a difference at that diameter.
Your width / offset & tyre diameter will be perfect
To be honest I didn't even know 205/45r18 was an available size.
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