Caffeine Zone 2 is a free iPhone/iOs app designed to get it right for you. Developed by scientists at Penn State University, you enter details about yourself and coffee consumption and it calculates optimal suggested consumption times and quantities. From The Age:
"The scientists said they used peer-reviewed studies to determine that caffeine drinkers are in an "optimal mental alertness zone" when they have between 200 and 400 milligrams of caffeine in their bloodstream. But going above this range can cause nausea and nervousness.
When they want to sleep, drinkers may discover that they have problems when they have more than 100 milligrams of caffeine in their bloodstream."
The app should be especially useful for those with irregular sleeping habits. The study was apparently supported by the US Office of Naval Research for this reason - the app will help sailors figure out how best to schedule caffeine into their ever-changing schedules.