A quick-and-dirty calculation you can do to get a very rough idea is, if you know the gross incomes, you just do simple math (in states which use the "income shares model" - which is most).
The income shares model is based upon equalizing the income between the homes - a RIDICULOUS methodology that often punishes - yes, you guessed it - the father. It is alleged that the children are "entitled" to the same theoretical income as if the parents had stayed together. Nevermind that they're not together and they now have to pay for separate households, but I digress...
If Dad makes $100,000 and Mom makes $50,000, in a shared income model state, half of the difference will flow to mom's household to "equalize" the income between households. That figure would be $25K divided by 12 to get a monthly figure.
Now, the calculations are typically done to arrive at a NET income figure after going through the CS Department's magical computer program, and is affected by who is carrying the healthcare, childcare, and perhaps some other expenses.
However, the illustration above gives you a very rough guestimate of how the process works and that's if the custody situation is 50/50.
From my perspective, it's not about the best interests of the children, in a majority of middle class cases, it's a simple transfer of wealth from the higher earning parent to the lower earning parent. Debate and discussion for another day.