But this delicate formula was overturned last month, when the Child Support Agency decided the maintenance should be increased to pounds 324 a month and the Treasury would claw back income support and other benefits from Susan.
Stephen, 34, from Rock Ferry, Merseyside, is incensed that one-third of his income is going to his ex- wife. He would be better off unemployed, as he would not have to pay child maintenance. He is seriously considering giving up his pounds 17,000 a year job - pounds 1,000 a month after tax - as a central heating safety inspector.
His new wife, Julie, 29, whose two daughters from her first marriage he supports, is also angry that their income will be slashed. Julie does not work and her former husband pays no maintenance for their two daughters aged nine and six who live with her, because he is unemployed and on income support. With the increased maintenance payment, Julie and Stephen Moore's income will barely match their outgoings.
And, even though her ex- husband will be paying far more, Susan is also unhappy because she will be worse off. As she is on income support, she is entitled to housing benefit, council tax benefit, free prescriptions, dental care and school meals. When the increased maintenance takes her above income support level, she will receive 80p a week more than now, but she will lose the fringe benefits.
Susan, from Bromborough, Merseyside, is seeking advice on how to challenge the new amount. 'The form made no mention that I could lose extra benefits . . . If I had known I would never have signed.'
Julie Moore said: 'People were led to believe the Child Support Agency was to force fathers who did not pay any maintenance to contribute. But instead, my ex-husband still pays nothing because he is on income support, Stephen's payments have more than tripled and his ex-wife will be worse off.'