Understanding Growing Degree Days

I'm still trying to wrap my head around this concept completely, hopefully some can help, but lets see what i understand this far.

Growing Degree Days - a measurement calculated to gauge the progression of plants through the annual life cycle.

In grapes, im assuming this is how they calculate bud break, flowering, fruit setting, verasion, and harvest dates (all approx.).The measurement itself is calculated by taking the average of the minimum daily temp & maximum daily temp.. And subtracting a base factor. The base factor is the temp at which the plants no longer grow - whats the base for wine grapes?

As a formula:
[(Min. Temp. + Max. Temp.) / 2 ] - Base Factor

The answer provided by the formula, is the Growing Degrees for that specific day. This is then added to the days following throughout the year, accumulating as a number from which to judge the progress of the plants and environment in question.

The things i know, i dont know:

1A. How many Growing Degrees does it take on average to get to the various stages of grape growing? Or are these different for everyone and something you actually dont learn until you're in the field?

1B. Do the averages vary from species to species, variety to variety, rootstock to rootstock, location to location... All of the above?

2. I dont know the Base Factor - the temp at which grapes stop growing.

3. How about information on the Growing Degrees needed for the vineyard pests to become real problems? Japanese beetles for instance.

4. Are growing degrees only calculated in Celsius, or does Fahrenheit work too? All the examples ive seen favor Celsius.

Theres probably more to this ball-o-wax, and probably a bunch i've yet to understand but hopefully this is a good start.

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