You are correct, corn Growing Degree Days (GDD), are similar to the requirements of tomatoes. Temps above 86F, and below 50F are not counted for tomatoes, just like corn. Go ahead and use the corn GDD daily updates for your tomato plan. (Since you garden in 3A, you do need a plan, ha ha.)

Here's a rough* guideline:
1400 GDD - small fruited, very early varieties produce breakers
1600 GDD - medium fruited, early varieties begin to break
1700 GDD - usually results in a fair crop for larger-sized early tomatoes
1750-1800 GDD - usually results in a very good crop for early tomatoes of all sizes
1800-2000 GDD - are the requirements for most mid-season (70-80 day) tomatoes
2000-2200+ GDD for later season tomatoes (81-day+)

*Keep in mind that GDD accuracy still depends on proper plant nutrition, adequate sun, adequate moisture, lack of disease/pests/damage. IE: If a hailstorm pounds your tomatoes into stumps, you can forget about following the GDD schedule.

As far as cumulative sunshine factors in, zone 3A has plenty of sunshine in the spring/summer with 13-16+ hours every day. (Unless, your zone 3A is on a mountain in the southern latitudes.) Always try your best to give your plants full sun. They can get by with 8 hours, but of course better results will be had with more sun per day. Shorting your plants on sunlight, would be like shorting them on nutrition or water and expecting they to produce an exceptional crop. Just not going to happen.

Good luck.

Thanks,

Where did you get the information? I want to add the source to my resource list.

Sun does make a difference. I've got the same type of plant on opposite sides of a row that runs north-south and should get adequate sun. The plant that gets the most sun has tomatoes, the other does not.

My source is 30 years in agribusiness, farming 2000-3000 acres.

It looks like this year I can expect around 1900 heat units. Growing degree days, 5C minimum, are around 1200. It would be nice to have some information to interpret the growing degree days. As to your estimates, my small early determinates have been providing eating tomatoes for a while, the sweet 100's are just starting to be productive, and I have loads of green tomatoes that will be picked that way. I think this matches your estimates. My tomatoes are usually smaller than reported by others or on seed packets.

I've included a couple of charts to illustrate the growing conditions this year. They don't say anything about a month of rain in July. Where are you located?

heat units at local weather stations from planting out

heat units and frost from May 1, 20011