It's crucial to consult your owner's manual or a factory service manual for the most accurate specifications for your particular vehicle. These manuals provide the most reliable information for your specific 1981 Datsun 280ZX. If you don't have those manuals, you might find this information from a reputable Datsun/Nissan repair resource online or at a specialized auto parts store.
Remember that fuel pressure is measured with the engine running. An inaccurate reading can result from incorrect testing procedures, worn components, or a faulty fuel pressure gauge.