I believe that thinking in terms of "what exact altitude do I stop thermalling" is bound to get you into trouble. Accidents seem always to be a chain of events, any link of which could have prevented the accident. They are rarely exclusively the cause of a single, sudden event. Focusing on a single event in an accident frequently ignores several other things that would have changed the outcome, even if the single event still occurred.

I think of safety in terms of my margin for error. This is affected by a great many things: first and foremost my appetite for risk, then in no obvious order pilot skill and currency, terrain, familiarity with terrain and aircraft, physical state of fitness at the moment, weather conditions, distracting concurrent events, mental state of mind, and many other factors.

I have picked my desired margin for error (it is higher than many pilots I know) and try to stay above it. If it is late in the day, turbulent, with terrain I will give up for landing very high. If I am fresh, have 5000 AGL, no other gliders around I might circle at 1 knot above stall/spin speed. These have a similar margin for error. Circling at 400 ft over a flat desert on a calm day has a greater margin for error than circling at 1500 ft in gusty conditions over terrain with two other gliders. A hard altitude number for circling is meaningless in isolation.

I have met pilots (who later died) who were skilled, but frequently flew with a very low margin for error. Most of the time they pulled it off, but one time, they didn't: the statistics of error probability exceeded the margin they allowed. I have met pilots (who later died) who were skilled, and even careful, but did not recognize the reduction in margin of error caused by some of their actions.

Of course most pilots instinctively or subconsciously try to balance the margin for error to some extent, and in any case it is not a number you can quantify. But if you think of it specifically as a quantity, and study even for a few moments what affects it, and keep it in mind as you fly, you are likely to change how you fly in some circumstances. Circling at 400 ft has already reduced your margin for error substantially in several dimensions. You can never predict or control all of the things that are going to happen: now even a slight distraction or small gust might exceed the margin you have allowed, and you become a statistic.