As far as I know the CM calculator doesn't take living in high cost areas such as London into consideration?
My friend's ex has recently gone through CMS and he now has to pay about £40 more a month which doesn't seem much but is the difference in affording the petrol and cost of feeding the children when he has them one weekend a month (6 hour round trip to collect and again to drop off) he moved back in with his parents who live in London after the split, over a hundred miles away from the children as he couldn't afford to private rent and had debts.
His ex wanted him to take them more weekends too but there's no chance he can even visit now which has devastated him.
Is an extra £40 really worth your children not seeing their dad just to get what you're 'entitled to'?