Concerning the SID monitor calibration,
is there any reason to worry that our daytime data are near
the -3.5 range and can get as low as -4.5 at the sunrise/sunset dips?
Do the monitors have a linear response throughout the range?
The monitors do not have a linear response to the way their gain
The SID graph in the daytime is like a hump due to the non-linear
effect of the Sun's angle and the layers of the ionosphere
reflecting the RF signal to your antenna.
The time of day that you adjust
the monitor has a lot to do with it. For example: 1 volt changed
in the morning will not be 1 volt by the noontime, it might be
more (everyone's monitor will be different).
Assuming that your antenna is oriented correctly
and no other RF sources of inference are to blame,
then you should increase the gain until you are in the -1.25
to -1.5 volt range around noontime.
If you can't make the adjustment at noon, then look at the
previous day's graph and try to estimate how many volts you need to go
up. Expect that you'll have to do this a couple of times as the effect is
not linear and overcompensation might occur.
(Avoid trying to calibrate your monitor at night --
the variability makes it too difficult!)
Proper gain calibration is critical and the good news is that once you get it
right then there is nothing else to fuss with!