[Solved] Child Maintenance - What should it cover?

Evening! I'm hoping some of you may be able to answer some questions for me - I am trying to arm myself ready for a dispute that appears to be looming, with my ex and with regards to child care and child maintenance. Some background...

My ex and I split up a few years back and have three children together all under the age of 10. Since this time we have followed the figures on the CMO calculator however have varied this to suit our circumstances, largely because my finances haven't been great, due to leaving the family home and taking pretty much nothing other than the debt that we had as a family. This has worked so far albeit has been financially difficult for me, but I am adamant that I will contribute towards the costs of my children. We do have a CMO case and it is renewed yearly but ultimately we pay what we agree with one another, and I have kept every message to evidence our agreements.

I contribute towards most costs; school uniform (of which they have plenty at my house, purchased by me, for use when with me,) school trips, school dinners, clubs and any childcare (childminder/nursery) which is used when the children are with me. They also have clothes, toys, gadgets etc at my house, again purchased by me, for when they are at my house. They don't need to bring anything from my ex's house, other than the things they want to bring. I provide everything they need for when they are with me.

I have my children for 3 nights (4 days) out of every 10. This actually works out (calculated hourly) as me having them for 33% of the time roughly - from mid day on the first day, all day second day, all day third day, until 7pm on the fourth day - 79 hours out of 240. This is the very maximum I can have them due to my shift pattern.

Going by the CMO calculator I should be paying around £400 per month. This to me seems utterly preposterous considering how often they are with me and the amount of money which I already contribute towards their on going care. Why do I think this? Because before we separated, we got by as a family on my sole wage of £1800 per month as it was at the time.

My understanding is that CMO works out the payment as being what is required by the paying parent to fulfill their half of the obligation towards the children - i.e. if they have them less than 50% of the time, they need to pay maintenance to make up for any time they have the children less than 50% to plug the gap. Which is fair enough because each parent should indeed be equally responsible for their children, and neither should be expected to be any more or less responsible than the other. Effectively the CMO calculator is saying that for the 15% of the time that they are not with me, I need to contribute £400 to fill that gap (i.e. from 35% to 50%). Based on this, the CMO calculator seems to think that to care for 3 children over 1 month costs in excess of £2600. Am I the only person who thinks that this is utterly crazy?

However, I suspect that this is actually not the case, and the CMO calculator quite simply is too rigid to fit in with the flexibility of many families shared care agreements, only takes in to account overnight care and doesn't give a monkeys that a father may also care for his children throughout the day. Luckily my ex has recognised (most of the time, when she hasn't got a bee in her bonnet) that this is pretty much impossible for me, and we have worked things out between us.

As I said things have been fine until recently, however her new partner has unfortunately been putting ideas in to her head that she should be getting the full amount from me. I have this on excellent authority from my oldest child...

What I am trying to ascertain is exactly what my child maintenance payments are supposed to cover. I am not at all interested in using my children as a weapon (which she has done before) but I need to be able to defend myself in this situation should worse come to worse.

If I am contributing the full amount as per the CMO calculator, should I be paying for other things on top of this such as school meals, school trips, school clubs, school uniform, general clothing etc?
What exactly does my child maintenance cover? What should she be spending it on?
If I am being expected to pay maintenance based upon having my children for a specific number of nights, then what it is to stop me refusing to have them during the day? (This isn't something that I would do because I love having my children stay with me and again I am adamant that I will fulfill my responsibilities.)

What if I was to say to her that because the CMO payments are based on me having them for only 2-3 nights per week (the equivalent of 3 nights out of 10) then I will only have them for the evening/night/morning and should would be responsible for them throughout the day. Effectively then reducing from having them for 35% (79 hours out of 240) of the time to 17.5% (42 hours out of 240). That wouldn't change my CMO calculation whatsoever, would it, because I'm still having them for 3 nights out of 10.

The whole thing is so shockingly biased towards the resident parent (let's face it, usually the mother) that it is quite disgusting. How the government can't come up with a system that can calculate a maintenance cost pro-rata, as is done in every other walk of life, is beyond me.