NASA’s James Webb Space Telescope (JWST) is a 6.5 m diameter, segmented, deployable telescope for cryogenic infrared space astronomy (~40 K). The JWST Observatory architecture includes the Optical Telescope Element (OTE) and the Integrated Science Instrument Module (ISIM) element that contains four science instruments (SIs), including a guider. The SI and guider units are integrated to the ISIM structure and optically tested at NASA Goddard Space Flight Center as an instrument suite using a telescope simulator (Optical Telescope Element SIMulator; OSIM). OSIM is a high-fidelity, cryogenic JWST telescope simulator that features a ~1.5m diameter powered mirror. The SIs are aligned to the flight structure’s coordinate system under ambient, clean room conditions using optomechanical metrology and customized interfaces. OSIM is aligned to the ISIM mechanical coordinate system at the cryogenic operating temperature via internal mechanisms and feedback from alignment sensors and metrology in six degrees of freedom. SI performance, including focus, pupil shear, pupil roll, boresight, wavefront error, and image quality, is evaluated at the operating temperature using OSIM. The comprehensive optical test plans include drafting OSIM source configurations for thousands of exposures ahead of the start of a cryogenic test campaign. We describe how we predicted the performance of OSIM light sources illuminating the ISIM detectors to aide in drafting these optical tests before a test campaign began. We also discuss the actual challenges and successes of those exposure predictions encountered during a test campaign to fulfill the demands of the ISIM optical performance verification.