You have your H's number, which sounds like he pulled out of thin air. Where did it come from? You are the one taking care of the kids, how the heck does he know what it costs?
Then you have the state number. Have you gone through working out the number you think is fair? It sounds like you have a lot of little expenses here and there which will add up. Forget the state calculator, what do you consider the cost of supporting your children? It might be worthwhile to draft up a budget.
For the b-day gifts, if he doesn't contribute, next time don't write his name on the card. That seems fair to me.
My take is both of your lives would be a lot easier if you just agreed on the lump-sum monthly amount, and you didn't need to have back-and-forth conversations over various expenses as they come up.
Perhaps you can consider raising that point first, before suggesting mediation as the method for coming to agreement on the amount?
