csa is usually 20% of monthly wage and for 1st child and extra 5% for each subsequent child so csa told me when i first had money from them don't know if it has changed but my payments are the samenow as then no idea if you work it out between you
I spoke to CSA about this the other day and it's currently 15% of the man's Net weekly wage for the first child, but it depends how much money he is earning, whether he has any other children living with him, and other factors.
Have a look at the CSA website....I think it's www.csa.gov.uk or similar.