FV/FM is considered the gold standard of chlorophyll fluorescence measurement because it allows sample comparison at the same known common dark adaption state and measurements generally correlate to photosynthesis measurement. While healthy values vary by species between 0.79 to 0.84, lower FV/FM values indicate plant stress is affecting Photosystem II.
For specific results on different types of plant stress measurement, ask for a copy of our Plant Stress Guide.