Final answer:
The exact percentage at which the sump tank full indicator illuminates is not specified and would typically be close to 100%, but the exact value depends on the system's design and manufacturer settings.
Step-by-step explanation:
The specific percentage at which a sump tank full indicator illuminates on the Beta-Freightliner R-12 Hydrant-Servicing Unit is not provided in the given context. Typically, such indicators are set to alert when the tank reaches a high level to prevent overfilling usually near 100% capacity. However, since this percentage can vary depending on the system's design and settings, one would need to refer to the specific operational manual or contact the manufacturer for the exact figure. Furthermore, information provided about ballast tank flow-through and sump pump drainage does not directly relate to the operation of a hydrant servicing unit's sump tank full indicator.