What we did to prepare was work out what the best case scenario of what we would seek, and the worst case scenario of what we would accept. For example, best case that we would seek was 50/50, worst case that we would accept would be 35/65.
We also structured some minutes of proposed orders to serve as a foundation for negotiations. That way, we had some framework to work around when discussing what each party would and wouldn't agree to.
We also created a brief document of the issues that we wanted to discuss, such as changeover times, time spent with on birthdays and Christmas, holiday time, etc.
Now, the actual experience included a lot of time wasted on back-and-forth with the ex about things that were outside of her control, like her wanting to include rules about when the child could meet new partners and who was allowed to drive the child. Those are day-to-day decisions and the responsibility of the parent with whom the child is spending time, so it was just an unnecessary discussion that caused more conflict than it solved, and wasted a lot of time that would have been better spent working out the major issues, like where the child lived and how much time she spent with the other parent.
Mediation is an inappropriate forum to discuss child support issues, and I wouldn't be at all surprised if the mediator declined to facilitate discussion about child support. The Child Support agency is responsible for everything child support, not you or your ex unless you're entering into a binding child support agreement, so don't waste everyone's time by trying to, I don't know, put a clause into your parenting plan that says 'The father shall be honest about his income to ensure child support assessments are accurate'. It won't happen.