2 or 3 mm is fine. The inside ring usually has the better chainline. On my conversion, it actually gave me a 42mm chainline. And you can use the existing 45. 45/15 sounds like a pretty sweet gearing for moderate hills/flat riding, but 45/16 would suit you better most likely.. a little lower and a lot more skid spots. And you will benefit from searching for a gear ratio chart to tell the gear inches of specific chaingring/cog combinations. Most people seem to gear about mid-70's but this depends on your riding style and terrain.