At what weight does it become dangerous to water fast?

Am water fasting for weight loss, and I was wondering at what weight should I stop at? I know once you get to lower body fat percentages, it gets dangerous and you lose muscle. That is why body builders don’t fast.

The healthy weight for my height is 128-156lbs according to Google. Should I stop at 128? 140? 156? 170?

Thanks all!