Yep, warm days and cold nights. There's other factors, real and imagined that affect sap flow too. I think it helps when the frost goes out, and weather systems seem to help too. A good snow storm coming through will stop the taps all at once, but when they open back up again it can be a heck of a run!
As far as marketing is concerned we have it pretty good. Growing veggies is our main business and my dad's been doing it for 40 years. We've been making syrup just as long. We stock our farm store with syrup and we've got lots of loyal customers who've been buying from us for decades. If I doubled my production I might have to get creative, but this year was actually the first year that we haven't run out of inventory before the next season. Last year was our first on vacuum and we made 180 gallons. About 75 more than the previous 5 year average.
Regarding finishing, we use a sap hydrometer to test for sugar content in the sap. It reads between 0-6 brix. We use a thermometer and a syrup hydrometer to test the syrup. When it boils at 7ーF above the boiling point of water, it's finished. Because we will cool, reheat, filter and bottle the syrup after it leaves the evaporator, I draw off the syrup a couple percent shy of the targeted density.