Child Support, how much should i ask for?

Sadly, a father of my 11 month child turned out to be a liar, irresponsible and unfaithful asshole. Last year, he left me with a 4 days old son to go for holidays with his daughter and parents to spain for two weeks. His parents are very privileged people and help him to look after his 10 years old daughter over who he shares the custody with his ex wife. His mum doesnt like me and told me that my ex partners daughter takes priority and she doesnt recognize my son as her grandson. My ex partner never had balls to stand up for me and his son so I made a huge step and decided to leave him. he pays child maintenance through CSA to his ex wife (they share custody and she gets �250 monthly as he hasnt tod CSA about our son yet) and ask me to agree to private arrangements. He stays with his parents and looks after his daughter half of the week so he wont have time to spend with our son, therefore i will get full custody. How much should i ask him for? how do i calculate it? Thanks for your help