IANAT but why would you narrow the band at all? Let's say questions have a difficulty rating one to ten. I'd start with a five, if it's correct, ask a six if not ask a four. Whenever they get a question wrong drop a one point of difficulty, when correct raise it one. In theory a person would end up bouncing around their proper area of difficulty. Figure out how long it takes a person to reach their optimal range and average their scores after that point. It'll take at least five questions even if you're perfect, so maybe it takes ten. In a twenty question test only the last ten would be counted in the average while the first ten would be making sure you had enough time to get in the proper range.