What would happen in your area with this scenario?
My area is the Netherlands, so probably doesn't help you. However, to generally answer your question, more on what I think is fair and reasonable.
Over here, there is no reward for looking after the kids more (except the joy of bonding more). What you do it add up all of the kids expenses (there are guidelines based on income, but you can also just itemize their expenses). Next you subtract any money received from the outside (typically government). Then each parent contributes to a theoretical pot their share of the remaining expenses; the share is based on incomes. The next part can sometimes cause confusion: for each of the expenses you determine which parent is paying the expense and add those up. From the pot, you allocate the money to each parent to pay those expenses. Now, in reality, as the final step, you work out what this means in terms of money moving from Parent A to Parent B, or vice versa.
Example (to mirror your case):
Expenses: 1600/month
Expense A: 100/month paid by Parent B
Expense B: 100/month paid by Parent A
Expense C: 700/month paid by Parent A
Expense D: 700/month paid by Parent A
I increased your child's expenses by 100/month to include their share of food/housing at both houses. Around here this would be the official way to go.
Received as tax credit by Parent A: 50/month
Pot: 1550/month needed -> split 85/135 for parent A and 50/135 for parent B
Parent A contributes 975/month Parent B contributes 575/month
Parent A pays for 1500/month in expenses
Parent B pays for 100/month in expenses
Parent A gets 50/mth from the tax credit and 1450/month from the pot = net of 475/month received from Parent B
Parent B gets 100/mth from the pot = net of 475/month give to Parent A
In your case, since you're paying the majority of the expenses (to be confirmed by both parties detailing their child related expenses), you would receive money even though to make more. Not because the kids are with you more often, but because you actually pay out the money for the expenses.
Parent B asking for money because Parent A sounds like alimony. Alimony is a completely different story, but it's easy for people to become confused. All money that is considered child support should be spent on the child (a small amount can be for their share of the living space, if you want to get down to details, but in general their food contribution should be what they eat, same with clothes, vacation, school supplies, etc.).
Last advice, work on open discussions and staying civil (even friendly, if possible). These situations are always made worse by bad interaction. Also understand that the situation sucks for everyone. Rare is the divorced parent who feels like their child support agreement is fair.