A mistake high is just a term used often in learning stunts. It is just the altitude needed to recover if you have a problem/mistake. Each pilot and every plane will have a different mistake recovery. When you read different writings on learning a new stunt it will usually say to try them 2 or 3 mistakes high. Something like the flat spin is learned way the heck high and the pull out is at least 3 mistakes high because some planes don't like to pull out of a spin. If the engine goes dead during the spin 3 mistakes high may not be enough on some planes.
Learning to use rudder doesn't call for a lot of sky, one mistake high like Ken mentioned may be about 100 feet or lower?