That said I do agree with him on not needing the water to be at zero because of the higher temps, if you are getting spotting then
the first thing you need to do is check that its not ingrained and therefore impossible to remove, next check the pattern of the
spotting as this will tell you whats causing it, runs of sptting down the glass will be water dripping from somewhere like vents
dodgy openers or seals, spotting all over the glass in different amounts will tend to be the fault of the shiner, cleaning too quick,
not using enough water, flow too low and so on.
Spotting is easier to see in full sun but you can be sure if you are getting it in full sun you will also be getting it when its not as easy to see.