I'm in the same situation - however I bought a visa late April 2012 (10 day stay) and returned (via Istanbul) July 2012 (12 day stay) on a 90 day/180 day visa. I forgot about this and bought a new visa at Dalaman in Sept 2012 (22 day stay) again another 90/180 day - so not sure that DLM are policing that strictly, as the May 2012 visa should have taken me to 24th October 2012.
Under the 90/180 rule, the September visa should take me 1st March, 2013
I plan to travel out end December for 10 days, and then again in February for 2½ months - therein lies the dilema.
My plan was to purchase visa for December (or try and use the September visa) and then purchase a new visa in February. I'm just confused as to who is using the 90 day rule or the 90/180 day rule. The December trip is direct to DLM, the Feb trip is via Istanbul.
Looking at the Turkish consular website in the UK it is still referring to the old 90 day visa and no mention of the 90/180 day one.
HELP :-)