Wasp - have to echo what others have said. What your OH LEGALLY has to pay has no bearing whatsoever on what his ex gets.
Your best bet is to get onto the CSA calculator, type in your ex's income and base it on that. If he wants to stip his maintenance down to the minimum - that'll tell him how much it will be.
If it helps, I'm a single parent to one child and used to get benefits when she was a baby.
I got approx £60 a week in Income Support.
£20 in Child benefit.
£45 in Child tax credit.
I also got HB and council tax credit, but don't like to count this as income as i'm not actually getting the cash iyswim? It went straight from DWP to the council.
My ex tried to palm me off with £5 a week. I went to CSA, had to get a detachment of earnings order and he now has to give me £30 a week.
Money is aboslutely shit when you're an LP to one child. After all my bills were paid i was left with £20-£30 a week to feed and clothe us, buy formula/nappies etc.
It must be even harder when having a mortgage; as far as i know, she'll be getting no help with that at all.
I'm now working and receive WTC. But i'd rather not disclose my current financial state if that's all the same.
Your post comes across quite cruel. But maybe that's me just being over sensitive because i've been where his ex has been.
Word of warning - seriously don't go below the legal minimum he's giving her. Use CSA calculator.
Think of the child, not her. It might make you more compassionate when coming to your final decision.