The amount is calculated on the number of nights that your children sleep at yours. Search for the govt CMS calculator online.
On those days you are normally responsible for collecting your kids from school, and their costs as well.
So if you do school pickups on Monday & Thursday, you should cover their school lunch, afterschool club or any activity costs on those days, and their breakfast club if they go to one, the following mornings. also costs like toiletries & out of school clothes & toys on those day. Your ex should do the same. Essentials like school uniform & shoes are shared costs.
Same principle at the weekend. On your days you pay food & activities.
Why does your ex claim you don’t do three nights? Is there something particular she is disputing?
My ex thought the same of me because he had no clue what was involved. In the end, I wrote down every single cost, – school lunches, food, clothes, shoes, toiletries, books, materials for homework, trips, sports, petrol – for a month, so that he understood.
You have years of co-parenting to get through so you both need to get this one sorted. Better than 10 years arguing & stress, with your kids stuck in the middle.