this will depend on where you live I suspect, as some people find it easier to get lower temperatures than others.
You need it cool enough that the snake's matabolic rate slows sufficiently so they don't burn up their fat reserves during brumation. A temperature in the 50's works well.
This year I took my snakes down to a low of 42, but mostly they were in the high 40's. They all came out of brumation fine, with no significant weight loss.
Skye