Bruce,
  The issue is not so much whether the Nikon ISO values are 'correct'
as much as whether or not the Weston readings will give you exposures
that are appropriate to the way the Nikon 'sees' things.  Meter
calibration is known to vary from manufacturer to manufacturer...ISO
values are a starting SUGGESTION and it is up to each manufacturer to
follow the formula in the ISO standard, but the ISO standard has a
Constant in the equation which the manufacturer choses to use!  For
that reason, Sekonic meters might yield a different exposure than a
reading in the same light taken with a Minolta meter, yet both are used
by pros.
  Add in the additional variable of what YOUR lens apertures really
behave like (optimistic, pessimistic aperture size in reality vs. the
indicated setting) and how your shutter behaves (slow, fast, right on)
in your camera!  So run some tests and decide for yourself how well
matched you Weston readings vs. your actual results compare to your
expectations!

--Wilt

Why not try it and see?  

Set your Weston to an ISO/ASA rating of 200 and set the shutter speed
and aperture combination to the exposure indicated by the exposure
meter.  Carefully examine the results, using the histogram feature and
by examination of the resulting files in your image editing software.

If you decide that the results are under- or over-exposed, adjust the
ISO/ASA setting accordingly on the Weston.

Job done!