The 85% Rule of Optimal Learning:
Very simple but very effective rule, if you're not failing 15 percent of the time when you're engaging in a learning experience, you're not maximizing learning. Or to put this another way, you know you've hit the learning sweet spot when you're succeeding at whatever you're trying to do 85 percent of the time.
So if you have the option to set the difficulty of learning or training, adjusted it to keep the learning success and accuracy at around 85%.
