Child support

[luvmabub]

I've been separated for over 2 years now. I have a good job and managed to buy my own home recently which I love
My question is about the amount my ex pays towards our daughters upbringing ( she's 12). When I left he said he'd give me the bare minimum calculated by the then CSA ( now ended )
This amounted to 10% of his take home pay since he has a son to his other ex
What I don't understand is how this is calculated to be fair
When we were together we used 100% of both incomes to bring her up
Now I use 100% ( quite happily and rightly so ). But he's allowed to throw a paltry sum at me and skip off and spend the rest of his ( much larger ) salary on himself
Who thought that rule up ??