Around me (The Netherlands) a lot of people seem to follow "the 7-5-3 rule":
- Find landable terrain at 700m, but continue searching for thermals.
- Start picking a field for landing at 500m, continue searching for thermals but stay around the field picked.
- Initiate the landing at 300m, ignore thermals. Congratulate yourself on a nice flight and focus on getting the glider down in one piece.

Roel