The theoretical max just uses your lat/long, time, and the atmospheric "transparency" value that you select for whichever calculation you choose.
I find that in general the transparency varies with slightly with time of year (in the UK at least). Being more transparent in winter and less in summer. I tweak the value slightly in autumn and spring based on my historic data.
And you are correct, the best time to make the adjustments is on a really clear day around the summer solstice.