Who is the 'paying parent' if care of children is 50:50?

I don't understand the government website child maintenance calculator. It starts the calculation by asking if I will be paying or receiving child maintenance. Good question! What are the rules on that? They define,
"The ‘paying parent’ does not have main day-to-day care of the child. The ‘receiving parent’ has main day-to-day care of the child."
What if we care for the children 50:50?
What happens if one parent does more child care, is therefore the receiving parent, but earns a bigger salary than the other parent?

Only you can determine, if you agree you are genuinely doing 50:50 then neither of you pay.

If it isn't 50:50 and one does more child care then the other parent should pay the maintenance regardless of salary.

But it's mostly your choice at this point how you wish it to be approached. Once you go down a more formal route if one of you disagrees for example then it's slightly less civil in most cases. If one of you does more care but earns more it's up to you to decide if you feel that the other parent should contribute more. if you both are contributing to the expenses relating to the children and are happy then the calculator could be meaningless in your circumstances and your own arrangements would probably be the most harmonious way