Once your knees are buggered, that's it, pretty well. Life gets a lot more sucky. So go low to go long. I have a touch over 13" on my 26" touring bike, to be sure, to be sure, but with 826% range so I still don't miss out on the higher gears. Even with derailleurs with a little work you can still get 700+% from 13" with almost perfect function.