At the suggestion of Al Kelly, I sent an email to Arne Henden of USNO.
Here is his response.
The equation Taichi Kato uses is
V = r + 0.375(b-r)
This is *very* approximate, as USNO-A will have zero point offsets
from field to field. It is better to use one of my deep BVRI datasets
to check your limiting magnitude. Two good examples up right now
are NGC7790 and M67.
I've never seen 19th magnitude visually. Note that the visual
will always fall between b (blue) and r (red) and not be outside that
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