A system is provided with an insulin delivery device configured to deliver insulin to a user of the system and a computer-based control unit associated with the insulin delivery device. The computer-based control unit includes a user interface and a computer-based processor. The computer-based processor is configured to calculate a relative insulin on board value for a specific time by calculating a first value that represents a reference insulin on board value at the specific time, calculating a second value that represents an automated insulin on board value at the specific time, and subtracting one of the first and second values from the other. The automated insulin on board value represents at least one insulin delivery automatically specified by the computer-based control unit. Methods of use are also disclosed.
