The system is due to change in October to be based on gross pay rather than net pay.
As a (extremely) rough estimate under the current scheme I would expect around £115 per week.
I based this calculation on a salary of £42k with deductions of around 30% for tax, national insurance, pension contributions etc.
For 2 children he has to pay 20% of what''s left. That makes around £5600 which divided by 52 is around about £115 per week. If he has the children less than 52 nights per year and does not live with any other children he has care of then there are no deductions to this figure.
I don''t know what the percentage of his gross pay will be under the new scheme, I''m not sure it''s officially been announced yet. If anyone would know then Fiona will!