Normalized difference red edge index (NDRE for short) is a method of measuring the amount of chlorophyll in the plants. The best timing to apply NDRE is mid-to-late growing season when the plants are mature and ready to be harvested. At this point, other indices would be less effective to use.

Sentinel = (B08-B05)/(B08 +B05)
Firefly = (B118-B072)/(B118 +B072)