Once you have your original goal achieved, a "GDD" number that is more accurate than the generic DTM, you could just add a column for "how close" the GDD number you pick for each variety came to each growers' actual experience.
If those "off by" numbers seem to correlate well with latitude, you have a good hint that growing-season daylength is a factor.
I don't know any way to compensate for "dry summer and I don't water consistently" or "my beds are in partial shade".
As you say, averaging over many growers' experiences should average things like that out.
>> I'm counting on finding data in field trials from universities and commercial growers.
That would be golden!
