The parent paying child care receives the child tax credit usually, and therefore that parent has to reduce the payment they make for child care by the child tax credit. Without any further information, I believe the $517 number is correct.
Yes, you are correct about spousal maintenance. If you pay her $500/mo, tack on $500 to her income, and subtract $500 from yours.
I'm unsure about the income tax calculator. Typically, I believe you would use the previous year's tax tables to determine the financial benefit by claiming head of household or for not sharing the income tax deduction. That benefit would then be applied on a prorata basis to reduce (or increase) the child support obligation. As far as a free calculator for this, I'm not aware of one, but you could probably get a trial version of the Bradley software, which may help you.