Child Support Calculator - What is calculated???

My wife is being threatened by her Ex who says he is to persue her for child maintenance 28 months after their divorce.

The Child Maintenance Options calculator is not clear and I need clarification on one point.

Can someone advise me. My question is.

1. When child maintenance is calculated is this based on which of the following.
a) Net pay (weekly calculated but paid monthly) after Tax & NI.
b) As above but also after deductions for paying into an employers share option scheme.

Currently my wife works part time 25 hours per week @ £6.25 per hour, pays little or no Tax or NI which works out at a Gross weekly pay of £156.25, or £625.00 Gross per 4 weeks approx BUT then pays £250 per month out of her wages into a share option scheme.
After paying Tax, NI and for her share option scheme she brings home around £375 net per 4 weeks, or £93.75 if calculated on a per week basis.

Does my wife as the non resident perent calculate the child maintenance to pay with or without her deductions for the share option scheme.

The child is now 17 Yrs & 2 months old, is in full time education for the moment and may attend Uni from Sept/Oct 2013.

Please help.

Andrew

Share option wont be an allowable deduction. CSA assessment allows for tax, NI and pension. Amount payable would be 15% of net.

Does she pay into a pension scheme?

Is share option a salary sacrifice scheme?

Does daughter stay with you at all over night?

CSA isn''t payable beyond A''levels