The scaling of the ARC-NDVI imagery:
I misunderstood the information that was sent relating to the scaling of the NDVI imagery. The formula should be:
I-NDVI = (400 * NDVI) + 0.5
This means that the values 1 to 255 cover ONLY the original NDVI values of zero to just less than 0.64 (which I have been told is greater than any value they have recorded).
Thus, the original NDVI value of zero:
400 * 0 + 0.5 = 1 (the software rounds the fraction up)
for the original value of 0.1:
400 * 0.1 + 0.5 = 40 + 0.5 = 41
For 0.64 (the maximum value):
400 * 0.64 + 0.5 = 256 + 0.5 = 257 (so the value doesn't quite go to the original of 0.64 it's actually about 0.6387).
Values of zero are flagged for water, cloud cover, missing data.