Telling my boss about my business

Because of my relationship with my boss and the nature of our work, I need to tell her about my business on the very early side. We are in no way competing, but my contract specifies I will not engage in any outside work. I will be asking for an exception to that. I work 50~ hours per week as a non-profit administrator and I love my job. My goal is to use some Saturday half days to build a law practice focusing on estate planning. My worries include: this will be a flat no, she will push back that if I have more time to devote to work, it should go to my job, she will question my commitment and plan to stay (which is very important to both of us). Any tips or best practices for preparing for this conversation?