Non-compete agreements are not illegal in California. You have been misinformed. Some kinds of non-compete clauses are unenforcable in some kinds of agreements, but you have not specified what kind of agreement contains the non-compete. There is no such "300%" rule that I've ever heard of in California or any where else, but an employee and employer could agree to such a thing if they wanted to.

If you are referring to a non-compete clause in an employment agreement, get the agreement to an attorney and get an opinion. Even if the non-compete is unenforcable, that does not mean you can steal your former employer's customers away from them. That is not only unethical, but it may prove very expensive when they sue you.