Last couple years we really haven't had turn over on most texas lakes. Just haven't had enough abrupt cold weather.
The lakes have mixed pretty well lately. Twok has already started,
To answer your question, there isn't any one temp...it has to do with how fast the upper 10ft water column cools and "pushes up" the lower 3rd water column. Since we haven't had abrupt cold weather last couple years, the lakes haven't turned over but have mixed. Mixing helps fishing stay consistent. Turnover hurts the fishing for about 8-15 days.