It's too hard to work out, which is why the csa sticks to a %. Then you get blended families like yours, where it's tricky to tease out how much one child is costing from a family budget.
Also one parents idea of what a child needs may be vastly different- violin lessons, if the mum wants the child to have them should dad contribute 50%, even if he doesn't think they should? Holidays, should a contribution be made based on dads cost for what he would choose, or mums? Some people would rather put the heating on than sit in a jumper....
Some people can spend thousands on their children easily, some can keep to a budget on benefits. Which do you use to decide?
One thing I will say is you can't really blame him that you have 5 in a 3 bed house if the other two kids aren't his? He shouldn't be expected to finance a 5 bed house for you.
Also his new partners wealth isn't relevant, just as if you met and married a millionaire he would still be expected to contribute to his children's upbringing.
Have you run the figures yourself to see if he is paying the csa amount? I'm afraid you can't insist he pays more (unless there is an agreement over private schools or something). My understanding is the csa can overrule court agreements, but it might be worth having a chat with a solicitor.