My ex earns a similar amount. He has the children 2 nights. He pays:
Just over CMS amount for 3 kids if he had them 0 nights (about 18% of his gross salary, which is about 24% net)
Half the mortgage (still joint until divorce, etc sorted)
That totals about 26/7% of his gross salary and 35% of the net one
He has never asked me for a breakdown of how I use the money!!
Once the divorce is sorted and the children and I are living in a new place/the mortgage on our family home is in my sole name, then he will no longer pay pay half the mortgage. He has said he may increase the maintenance payments so that actually he will still be paying the same total amount as he is now. We will see - I cannot legally hold him to that but I believe he will stick to it. As he says, as long as he can afford it why wouldn't he want to support his children (not me) as much as possible. If my income goes up, he couldn't care less as it just means our children get a better standard of living, as it were. (If his income goes up, he will pay more, if it goes down of course he will pay less).
My ex is not an arse. In his view (and mine) the children are a joint responsibility in all ways and his financial contribution should not reduce to the bare minimum just because he no longer lives with them. He should still contribute as much as he can to give them the best standard of living, 24/7, as he can.
I cannot understand mothers who think differently from that and think it's okay for a father to only contribute the minimum required to keep kids fed, clothed, and housed regardless of his income. When the adult relationship ends the adult-child relationships do not end and neither do the adult to child responsibilities.
No decent father earning £1300 a week who lived with mum and his children would only put a tiny fraction of earnings into the family pot. A relationship where that happened would be clear financial abuse. When parents split up, finances need to be stretched further but that doesn't mean the NRP/father's financial responsibilities should become the bare minimum!
OP - I think your suggestion is fair but I guess he doesn't have to agree. I guess giving him a breakdown of food, activities, clothes, childcare, etc, every last little thing might make him more likely to agree if he has no idea whatsoever how much this all adds up to? I'd not be happy about having to do that but probably would as a compromise, as long as he wasn't going to keep requesting it/using it to try to check up on my spending in some way.