I'm going to convert an older Trek to a singlespeed for my wife. It has a Shimano 8-speed groupo at the moment. I'm planning on keeping the rear wheel, so I need to figure out just how much distance I need spacers to cover. I'll be keeping the spacers that are part of the cassette and I'll be keeping one of her sprockets. Is there an easy way to figure out how much space is left to be filled in before the lock ring?