Apologies if i am repeating, I have not RTFT.
The problem actually lies with the large % of parents (be it mum or dad) who the child(ren) live with for the lesser amount of time (if any) who do not want to support their children. Once they are separated they do not want to give money to their ex partner - this is part of the problem they see it as giving the other parent money, and their mindset is not trained to think I must financially support my child.
Money is always a sensitive matter, there may have been acrimonious divorce / separation proceedings and this then continues into the maintenance aspect.
If the mindset of parents was different then the services of CMS would not be required. Except maybe in some certain situations.
However, again if the mindset of parents was different, I.e. they paid CMS amount, did not put barriers up to enable the CMS to do their work, we're not difficult / uncontactable then again the service provided by the CMS would probably greatly increase.
With regard to high earners, whereby, the RP definitely knows the PP earns more from the time they were together- due to their bills, spending, holidays, cars lifestyle etc. Yes in theory they are 100% correct, however, the way in which the PP probably declared their income (ie not all of it to HMRC) highly likely hasn't changed since the time they were together as a couple - it more than likely wasn't discussed and maybe the RP was unaware of this. Or if they were they suddenly want the CMS to be able to prove this.
Turning to what is the fair / right / correct amount for the PP to pay to support a child - well who knows?? Even the CMS has changed the way they calculate maintenance over the years.
A PP very early on said it should be a minimum of £300 per month per child. This may well be unaffordable, I know I certainly wouldn't like it if I was told that everyone will pay the same amount of tax regardless of what you earn (maybe not a great example- but couldn't think of another). So one person earning £18,000 p.a will pay the same as someone earning double them??
Hence I believe (from reading other threads) on here that's why, it is a % of the PP earnings, the same reasoning behind how we pay our tax - a % of our income, relative to what we earn.
I may think £50 per week per child is fair. That's roughly £400 per month per child assuming the RP matches the PP contribution, if as so many people say it should be half of a child's needs paid by the PP. Someone else may think it should be £100 per week per child, that's £800 per month for the child's needs.
What should be included as part of a child's needs ? Childcare costs ?, clothes, toys, uniforms, school trips, activities, gadgets for older children?
Childcare costs - MN consensus seems to be that childcare costs should be split to enable the RP to work, not fall behind in their career. What if, when the child is young PP also needs Childcare when they have the child with them. What if the PP doesn't need child care, but would like to pay for an activity whilst the child is with them? How would that work?
Reduction for children living with the PP - this is always a divider. If happy couple have child, and can afford a certain level of lifestyle, but when a second child comes along, they maybe can't afford a takeaway twice a week, now its ony twice a month, or instead of spending £100 on new shoes, it has to be reduced to £50, they have to reduce their outgoings to enable them provide for everyone. In certain ways this is similar to a biological child of the PP with a new partner. Just because child 1 was here first, doesn't mean they are entitled to more.
A little more difficult is the PP living with children of their partner, whilst in theory they are probably providing for the child(ren) - should a reduction be allowed? I honestly don't know what the right answer to this is.
I don't think there will ever be a right / wrong, fair or not way of working out how much a PP should pay.