I'd concur with some others that you are a diffuser heading to a NW5....NW6 level; as mentioned, the NW scale, while not worthless, simply isn't a beacon of truth for many, but it does have its uses.
As you've stated, though, we are seeing your hair in basement conditions. You might currently be something like a NW4.
Regarding predicting your future loss, I've been of the assumption that looking at your hair in "poor" (i.e. "truthful"...) conditions is ideal....I've been checking out mine recently under harsh, bright lighting from a top-down view and it is downright frightening! ......... ..... ..... ......