You're right about HDD calculations - HDD databases use 65F as their base temperature, and assume 10F of gains from lighting, appliances, inhabitants, etc. Bases can be adjusted to reflect improved insulation, different temperature setpoints or different internal gains. Base-65 HDD tables are still a useful tool for simple calculations (like the ones we're doing), which is why they're the industry standard.
I checked the data I linked earlier (admittedly the result of a quick google search) against the
ASHRAE tables (copyrighted, but available at your local library), which also provide city-specific data. Miami's annual total in 2007 was 141, all locations listed in Hawaii totaled 0 annually, and Alaska's average was in the range cited in the last link, with Fairbanks having an annual total of 14,279. So I'd say the previous link's accurate enough for our purposes.
I checked your math, and your calculations are fine - and your result agrees with the "heat for 16-20 years" figure you quoted earlier. My point is that it's a misleading figure, for two reasons. First, the gas in that reservoir won't be used to heat homes in Florida (at least not more than a few percent of the 2.6 tcf). It'll be used for one of two purposes, depending on regional gas prices: 1) to provide peak power to major urban centers (which may or may not be anywhere near Florida), or 2) to heat homes (again, likely not in Florida). I used Miami electricity supply as a counter-statistic because it's a more realistic use of that resource.
Second, saying you can provide heat for Florida is just kind of vacuous - it's like saying you can provide for all the air conditioning needs in Antarctica for a millennium. Okay, that may be true - but who cares?
All that being said, I'm not strictly opposed to drilling that reservoir (I'd have to learn more about the economic and environmental implications to take a firm stance one way or the other). Natural gas is a relatively clean fuel, and it's extremely valuable for meeting peak electrical loads. As intermittent power sources like wind and solar get integrated into the grid, the need for peaker plants will only grow. Natural gas isn't going anywhere, and I agree that we might as well get it from domestic sources.
I just think we should weigh the costs of domestic drilling (in Florida or elsewhere) against the
actual benefits - not some misleading stats delivered by the people who stand to profit from exploiting those resources.