My head hurts - CM what covers and what extras?

My head hurts from battling with my ex over what is included in CM and what 'extras' are normal for NRPs to contribute to?

Are there any authoritative opinions? I can't find anything despite months of looking, and even past threads here don't throw anything up.

Can anyone help?

As background:
I'm looking for something that will provide a benchmark for us to work with.
I want to pay what is right for my kids but I increasingly feel that I'm being used as a walking wallet so that she can work less and spend more. I already resent what I see as my being scalped on the divorce settlement and her withdrawal of contact (horrible word) if I don't cough up on demand.
I pay the full CSA rate of CM (it was the full Consent Order rate until she called in the CSA, only to be hoisted on her own petard when the CSA rate came out lower)
Although she earns much more than me, I have agreed to pay half 'extras' such as school trips, school uniform.

Pay the cm,pay half the other things if you can afford it it's not obligatory
All the best
Pete xx

CM is a contribution towards the costs of raising children. Your only obligation is pay CM at CSA rates and it is entirely up to you if you wish to contribute more voluntarily.

CM is what you are legally obliged to pay. Nothing more, nothing less. Of course, any decent father would want to help out with uniforms, trips, etc, but technically that is what CM is for.
Emotional blackmail is par for the course.

My ex pays me £5 a week CM, and no extras. To him, extras include birthday presents, so no, she did not receive a birthday present from him.
He did buy her some strawberry hard boiled sweets last week, but seeing as she is only two and allergic to strawberries, she couldn't have them!
My ex earns £700 a month but has managed to get himself only paying £5.

Like the others I understood that you were only obliged to pay CSA rates and it was voluntary if you pay for anything else.

I use CM to pay for school dinners and bus fares. In the school holidays I put the money aside to pay for school uniforms and it covers about 40% of cost. My stbx had said he would also pay half of any school trips but money is never received and I'm not aware of any way of making him pay it. Personally, I would rather do without something myself to cover the cost of the trips as the fallout from mentioning he agreed to half cost of trips is not worth it.

So pay CM and if you have any extra maybe buy the kids something when they are with you that way you know it is getting spent on them.

Good luck:)