chris - "filters ... to adjust the color sensitivity of the meter to be closer to film"... First of all, WHICH film? Yeah, Fred Picker pickerfied Tri-X 320, that much we already know. But even panchromatic films differ in specific spectral sensitivity. Even TMax itself differs somewhat in red sensitivity between its first and second generations. Current ACROS ii differs in blue sensitivity from the prior version; and both of them are Orthopan (diminished red sensitivity, and not true panchromatic at all). So you've got that kind of issue. And am I really supposed to believe that an ole medicine wagon snake oil salesman knew better than the Pentax engineers? Well, I'll let Richard Ritter decide that one. But I've encountered both opinions among Hollywood cinematographers, at least in a random sense. I doubt anyone has ever done a survey. One just gets accustomed to the idiosyncrasies of their own meter over time. But the modified version does have a potential strike against it in the sense the supplemental filters fade over time; and that can't be corrected with a simple recalibration procedure.
Ad far as flare goes, I just attach a collapsible rubber lens hood to the meter just like with a small camera lens, and avoid aiming at either directly at the sun, or a specular reflection of the sun. And it might be interesting to compare the Soligor spot meter, which is not multicoated like the Pentax.
Thanks for providing your results. That should be a help to those trying to understand the basics. And Pentax published its own spectrogram of the sensitivity of their meter, which is relatively symmetrical on either side of the green peak.
I also do tests using the same MacBeth Color Checker chart, but with respect to photographic mean daylight (coior temp meter accommodated). I don't trust LED lighting, though I did buy a relatively expensive set for sake of my copy stand. Then I make specific sheet film exposures of the chart, develop those, and take densitometer readings of each patch. Or I did. Got all that filed away in binders. Now everything is largely intuitive anyway. Spd meters are now pretty much standard. Old Cds ones aren't in much use anymore.
Something else that hasn't been mentioned yet is that Fred Picker had the habit, and taught, metering through colored black and white contrast filter, which, as far as I'm concerned, can be a minefield, especially if different films are used. It's a topic no doubt with its own past threads; but it's a darn poor and potentially leading substitute for intelligent use of tested filter factors per se.
But it colored his notion of how a meter should be "improved" - again, improved with respect to which film, and "unimproved" with respect to which? But the patent medicine man said, "Just drink it; it's the elixir to cure all your ailments". Yeah, I spent a fair amount of money with Zone VI myself, and some of Fred's products were great; others, well ... not so great.