May I suggest that you put the bike in the gear you think you will use and try it for a week before you decide the big to little is what you want to use. Most single speeds seem to work best with a gear of about 65 to 75 gear inches. Your proposed gear will be more like 104 gear inches and unless you are built like Lance you will not be happy with it. Try different gears until you find the one you like and then cut the chain. 72 gear inches is for example 40 tooth front to 15 tooth rear sprockets. Good Luck with it. Roger