I saw an interesting guide recently, never had seen one like it before. It was a plastic rectangular plate that had the tooth angle inscribed on it, and it attached to the bar beneath the chain with a couple of imbedded magnets, that held it out a bit. It was easy to mimic the angle on the plate with your file, and it didn't require any more motion or time compared to not using any guide at all, my usual method. You dont even have to line up the chain with the disk, just an eye on it gives a reference for a number of teeth until you slide the chain along. It appeared to be a very good idea. Anyone else seen one like that? Every once in a while using it seemed like it would keep your angles true.