It's all relative.
You may be the only dad you know that pays SM to a high earning XW, but there will be others.
Whatever she said in court about not working FT, it's irrelevant now.
Yeah, she may have lied to get the best deal - which seems to be what you're implying? But that's why the court is there - the judge is allowed to say, actually you should work FT.
Now once the divorce was all done, perhaps she thought - bloody hell, I'm on my own now, £350 a month isn't going to cover me now let alone get me a pension. Actually, I need to work.
My childcare fees for one child are £250 a month - before and after school club.
What are the childcare fees for your 3 children? That have to be paid so she can work. And why should she pay those alone?
Do you see where I'm coming from? The £900 was for housing, clothing, feeding etc...
She could easily be paying £700 a month in childcare now making your £350 a fair half.
You could even argue that you should pay more than half, because it's her "turn" to have her career opportunities supported.
I don't know who is reasonable and who is not, but to even begin to decide i would want to know who is paying childcare and how much.