Being in the Northeast, I've wondered which citrus varieties have a reasonable chance of maturing properly if planted in ground during our growing season. I recently stumbled across this excellent website that will calculate GDD for any location in the US over the past few years. The website is a little buggy, but still provides a wealth of good information if you play around with it.

Here's the link: http://pnwpest.org/US/
You can manually select your upper and lower temperature thresholds for the calculations. I've been using 55 and 95F (or 13 and 35C) for citrus and and the "simple average" calculation method.

Unfortunately, there does not seem to be much data on the internet regarding the specific GDD requirements for different citrus types (e.g. grapefruit, orange, satsuma, lemon, etc...)

Here's one paper that gives some ideas, but nothing too detailed:
http://irrec.ifas.ufl.edu/flcitrus/pdfs/short_course_and_workshop/citrus_flowering_97/Goldschmidt-Effect_of_Climate_on_Fruit_Development.pdf

Nevertheless, you can compare the annual GDD for your location with those of other areas where citrus grows outside without protection. For reference last year, I had ~2000 F GDD (and ~1100 C GDD using 13C as the lower temp threshold) This is more GDD than many locations in the SF Bay area, and is on par with commercial citrus growing areas in New Zealand (see paper above). I was also surprised to find that my GDD are not that far behind some of the more coastal commercial citrus growing areas of southern California such as Santa Paula (~1250 C annual GDD). Of course none of this takes into account the length of the growing season, but if one is able to protect their in ground trees over the winter, I believe this becomes less of an issue. So for me, if it grows and matures in the SF Bay area, select coastal S. Cal. locations, or New Zealand, I should be able to grow it here outdoors and get it to mature properly with appropriate winter protection.

I found out about the "citrus daily heat units" calculation myself a couple of weeks ago, I guess it was on this forum. Here's another article...

http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0011/218972/Calculating-heat-units-for-citrus.pdf

As it happens, I have built a microcontroller based weather station. It doesn't do any measurements itself but instead downloads airport METAR and TAF data from the internet and displays it on an LCD screen.

METAR data is produced by airports and contains weather observation data, including temperature. Likewise, TAF data contains weather prediction information.

I recently built a new function into my weather station to collect temperature data over 24 hours and use that info for DHU calculation. At first I used the simple average method described in the article, but I don't find that a very correct method. I just now implemented another way to calculate DHU. It now actually integrates al temperature values over one day. That should in my opinion result in a more reliable value.

Yes, I saw that document a while back too, I believe Millet posted it. It sounds like you have a pretty good setup for DHU calculations. The link I posted also uses METAR data, along with a few other sources, and is quite nice because it allows the user to select the lower and upper temperature thresholds. The added bonus, at least for me, is that one of the reporting stations is less than a 5 minute walk from my house. Most GDD/DHU data in the US use 50F as the lower temperature limit instead of 55F.