Before items can be implemented in a test, the item characteristics need to be calibrated through pretesting. To achieve high-quality tests, it's crucial to maximize the precision of estimates obtained during item calibration. Higher precision can be attained if calibration items are allocated to examinees based on their individual abilities. Methods from optimal experimental design can be used to derive an optimal ability-matched calibration design. However, such an optimal design assumes known abilities of the examinees. In practice, the abilities are unknown and estimated based on a limited number of operational items. We develop the theory for handling the uncertainty in abilities in a proper way and show how the optimal calibration design can be derived when taking account of this uncertainty. We demonstrate that the derived designs are more robust when the uncertainty in abilities is acknowledged. Additionally, the method has been implemented in the R-package optical.