Hi
I'm not sure if you have been using the csa for a while but I know they now have a new system, as I have recently spoken to them.
On the new system it does not take into account what income you do or do not have but solely what your ex earns.
For you it will be 20% of his net WEEKLY Income after Income Tax and NI.
They see the process as 'they are ensuring you get what the ex would have contributed to the household if you had not split' (their words not mine)
Have a look at this link it might help explain things a bit further.
http://www.csa.gov.uk/en/PDF/leaflets/new/CSL3 03.pdf