Since your need seems to be to figure out "when to start saying ותן טל ומטר in diaspora", I'll just focus on that (and ignore the mention of the tekufah in the title):
To simplify things, start with the date November 21.
You will then need to adjust based on two rules:
- If the following year is a Gregorian leap year, push the day off by one (i.e. November 22).
- Then add days based on the drift between the Julian and Gregorian calendars [see the chart]. This drift increases by 1 day every 100 years, except where the 100th year is divisible by 4 (e.g. 2000).
For example: This year (2011) is the year before a leap year. It also happens to be that we are running a 13 day difference between Julian and Gregorian. So take November 22, add 13 days, and voila: December 5, 2011 emerges.
And you should definitely read this for more.