it comes down to a lot of factors - primarily - the income of BOTH parents supporting the children. if child 1 is to a mother who is unemployed, and Mr X has high income, no shared care etc, then he would pay a higher amount. if the other three children are to his current partner, his responsbility to them (fiscally) is less as both parents have to contribute (to the house etc). it really is a very messy calculation. if he has between 14 and 35% shared care of child 1, his CS responsbility is reduced by (i think) 25% - as i said - messy messy calculation!