There is no need to guess with chain line. If the new crankset specs 45mm with a particular BB then you are already half way there to getting the chain line correct. Sheldon Brown shows you how to calculate the rear chain line for your bike. Figure that out. Is it 45mm? If it is then use the stock setup. If it isn't then you need to add or subtract BB spindle length to get the same chain line at the front. Of course you have to take clearances into account when going shorter and FD chain line specs if you are working on a multi-speed bike. Al