... the meter appears to be off a bit...
In what way? Originally the meter was simply a relative output indicator, it isn't
calibrated at all. (And, if it were, the scale wouldn't be linear in watts.) You just
use it to adjust the loading control for maximum, that's it. It measures RF voltage,
not power, so with a high impedance load (like a full wave loop) it will read higher,
and a lower impedance load (like the same loop fed through a quarter wave of
coax) it will read lower for the same output power. Don't expect anything more
from it than that (unless you change out the power detector circuit.) It isn't any
good for adjusting a tuner without modification, either. (I added a power meter
to mine so the meter reads the difference between forward and reflected power.
Sort of... It isn't perfect, but that way I can just tune everything for maximum
output, even if I don't know exactly what that is.)
The S-meter mod was basically to connect it to the receive audio in the same
manner - it just measures the audio voltage (on the top of the volume control
if the mod was done correctly.) This gives you a relative indication of stronger
or weaker signals (depending somewhat on the tuning) but again isn't a precision
instrument. I just estimate signal strength by ear by how far I have to turn down
the RF gain control.