At what level of taxation does a company stop working for the shareholders and start as a State owned business?

If the share holders get less in dividends than the Government then something is wrong. They may invest in something else.

If the business is a private small business, the owner will look at the remainder of his income verses the percentage of profit from his investment and the massive amount of hours required to keep the business going.

It is up to the individuals involved. Everyone has a threshold of tolerance but obviously there is a point at which they will think it isn't worth the effort and send their manufacturing off shore so they can sit back and collet something more aligned with the work than investment they provide while their former employees are out of a job.

In theory, 100%.

However, we are nowhere near that.