I always see the paradox mentioned, at what percentage the tolerant actually flips to become the intolerant?

Are you asking, at what percentage DO the tolerant actually flip to become the intolerant? This is a good question, though probably impossible to answer with any precision.

I fear that it happens a lot more frequently than most people are willing to admit.

Eg. The nazis could argue to themselves that they are just getting rid of the intolerant.