You choose the perfect ratio now. In twelve months time, if you've bothered to think about your technique and fitness, it may well be wrong, particularly if your riding habits have changed. Then again, it may not have. Mate, it's trial and error but most importantly, it's all about reading your body. Forget the numbers. If you feel over geared, add a tooth to the rear. If you think you could go a higher gear, try one tooth less. These parts aren't expensive ... unless you're buying White Industries, which you don't need to do until you're comfortable that you've found the right gearing (and not even then if the truth be known).