My son’s father is self employed as well, and I have no idea of what his actual earnings are. I basically just nagged and nagged about it by email until he eventually agreed to send £185 a month, he often complains about it but I just ignore it. I don’t know if I should be able to get more, but at least it is something I guess.
My only advice is keep nagging him, what have you got to lose? It is hard to go through CMS because he is self employed and that always put me off that option.