Richard, I am somewhat in your boat, but the definition I was always presented with is .65mm (25.6mil) and smaller is classified as fine pitch.  This is the definition used by Topline for their dummy parts.  I have also seen this number floated about in several papers and equipment manuals.

For lack of clarity, I would propose this as a good guideline based on IPC inspection criteria.
That is, once we are at 0.65mm (and lower), the lead width itself, by necessity, reduces to 10 mils (0.25mm) give or take.
At 10 mil (0.25mm) lead width, Table 1-2 of IPC-A-610 jumps the required inspection magnification from (7.5X to 10X) up to 20X for inspection.