AIBU to ask for CMS amount plus half of nursery fees? What do other separated parents do?

[ThisQuickOliveCat]

My ex and I are separating and have a 2-year-old son. We both earn around £50k and are planning a 70/30 care split, with me having majority care.
My ex has offered to pay half the nursery fees (£275/month) plus a contribution towards our son’s share of my household bills and living costs. He’s asked me to calculate and itemise what I think is fair.
CMS estimates around £340/month, so I was considering proposing £340 plus £275 nursery, totalling £615.
Just to clarify, I know CMS includes nursery and that he’s not legally obliged to pay anything on top. He’s voluntarily offered to contribute more, and I’m simply trying to work out a fair arrangement for both of us.
I’m struggling with how to calculate a toddler’s share of household expenses. Do you include a proportion of the mortgage, council tax, utilities, food, etc.? And how do you decide what percentage is reasonable?
I’m not trying to get as much money as possible. I genuinely want a fair arrangement that reflects our equal earnings and different levels of care.
For those who’ve been through this, how did you calculate a fair contribution? Would £615 be reasonable, or would you approach it differently?

I’d frame it as the nursery bill is only temporary so the standard cms and half the extra nursery.

I think it's very fair for both parents to pay half of nursery fees allowing both parents to work plus whatever CMS calculator says. It's only a few years of the additional nursery payments and it allows both parents to work.

Can you just say £600 (including nursery fee)? Tell him what CMS have calculated he should be paying…. See what his response is

The CMS calculation is the child’s “share” of living expenses. Or rather the NRP contribution to living expenses.

My ex and I did split the cost of the kids school buses on top of this so I suppose nursery fees would be similar.

That figure sounds fair, and can reduce when nursery finishes.

I think it sounds fair but I’d probably mention now, half of nursery, but when your child goes to school, this will be half of the school uniform costs plus half of any extras like school trips.

I agree with this.

taking into account your fairly equal incomes and your 20% extra care £600 is approx 20% of his wage to cover those living expenses your extra time and the nursery fees which will allow you both to continue working.

as far as the rest is concerned you should neither be providing him a home, food whilst your caring for him and clothes toys etc at you’re homes for him.

Also agree once nursery ends school uniform, clubs etc should be split 50:50 and you need to figure things out like how you decide what clubs he attends and how it works around your schedule.

with proper and effective co parenting and a reasonable ex your child can live a life whereby he has equal experiences in both homes from each parent.