There are a few basic universal truths for hay harvesting. The problem is we do not fully understand many of the basic truths.

Truth No. 1  The equilibrium moisture content of hay is proportional to the relative humidity in the air and the air temperature.

This is the all important truth for any hay harvesting. Local accommodation for this truth is where all the controversy can be found.
It is well known that hay at 40% moisture content has very little leaf shatter, but can mold in a bale.
Hay can safely be baled at roughly 18% moisture for small square bales, 16% moisture for round bales, and 14% moisture for large square bales.

The moisture levels of the hay is effected by the relative humidity in the air next to the hay. The effect is, with a 90% relative humidity the hay moisture will be at least 40%. (1)
This same chart has the hay at 18% – 20% moisture with a humidity at 70% at the windrow. I find hay that is fully cured will be safe to start small square baling when my humidity measurements show 65% humidity at the windrow. The hay will have significant leaf shatter when the humidity is down in the 55% to 60% range.

I understand for night time baling the humidity is usually allowed to go to 65% to allow the moisture to get into the leaves. The time to stop is when the hay becomes too tough to bale. This early night dew can supposedly register 40% on the moisture tester and still be reasonably safe from mold. This is something each individual will want to work out for their self.

This same bulletin has some interesting information on the build up of sugars during the day.  The trick is there must be enough sunlight left in the day to dry the hay down to 48% moisture to stop respiration and the burning of carbohydrates over night. (2)
What this is telling me, if I start mowing at close to 11 AM DST and quit by roughly 3 PM DST I will gain a few percentages of digestible energy in the product.
Another reference (3) is full of useful information. Page 9 has a world of information. It will illustrate how a wide swath will improve curing times. With a little judgement we can have a good idea what day we will be able to bale the hay. The difficulty here is forecasting the pan evaporation. There should be numerous sources. I use the Forecast from www.awis.com agriculture and hay making service. (4)

(1) Figure 5 in  http://www.wvu.edu/~agexten/pubnwsltr/TRIM/5811.pdf
(note the data relating to figure 4 is mostly out of date)
(2) Figures 1 & 2 of http://www.wvu.edu/~agexten/pubnwsltr/TRIM/5811.pdf
(3) Page 9 of http://utahhay.usu.edu/files/uploads/UHFS07%20pdfs%20for%20web/Undersander%20Swathing%20and%20Conditioning.pdf
(4) http://www.awis.com/wx/tx/hay.html
Added reading:
http://www.uaex.edu/Other_Areas/publications/PDF/FSA-2005.pdf
http://www.uwex.edu/ces/forage/pubs/drying_forage.pdf
http://www.pubs.ext.vt.edu/442/442-454/442-454.html
http://www.pubs.ext.vt.edu/442/442-454/442-454.html
http://alfalfa.ucdavis.edu/+symposium/proceedings/2008/08-235.pdf