Agreeing maintenance payments with exP

[HowLongIsTooLong]

Have been losing sleep over my financial situation so thought I would seek some advice here.

ExP and I have two DD. We first separated two years ago and after couple counselling and various attempts to work at things I have decided it really is over. I currently live in his country (in southern Europe) and have a work contract here until October, on a pretty basic salary. He travels all over the world on apparently high-paid freelance work though nowadays I have no clue what his actual take-home is, but am pretty sure it is substantially higher than mine, but it is irregular, in that he only gets paid when he finishes assignments, and he has a track record of being late with stuff.

Since he moved out two years ago I have the DD living with me at the property we bought together 6 years back. He has the girls 2 nights a week when he is around (he can be away for 3-4 week stretches). We have both been muddling through and covering the biggest expenses together (mortgage and school fees - DDs are both in private schools due to the failing educational system here, long story, but one DD will be going to a free state school as of this September). The situation feels really messy and chaotic and, together with the emotional fall-out from the separation, is really getting me down. We need to sort out a system whereby he pays me an agreed amount of monthly support so I can plan and budget and stop mulling over things constantly. At the moment I find myself on the edge at the end of every month and he has even asked to borrow money when he has been late delivering work and has not been paid.

None of this is helped by him clearing a large amount out of our joint account when we first separated - a source of great anger and resentment to me - which is another issue which needs to be resolved when we sort out the finances. So far, every time we talk about money it turns into a huge conflict (aggressive on his side - one of the long-standing tendencies which made me leave him) so I think this has also prevented me from pushing harder to sort this out. I am also pretty sure he has been lying to me about what he earns (while proudly telling others about his daily rate - grrrrrrr...what an idiot)

Anyway, while I need to be able to plan my finances, am terrified at the prospect of a future life of poverty, and wonder if that is holding me back from actually getting a grip on things and facing up to our appalling situation!
My thinking is that we should carry on paying the mortgage 50:50 as we have equal investment/equity on the property and no plans to sell up as yet, but I need to figure out how to calculate a final monthly maintenance from him, and where to start negotiations on it.

I saw a lawyer here and she told me that the usual system is that the school fees and health costs of the children are split 50:50 and then couples need to figure out how much the father pays to the mother, which is based on relative salaries and how much time each parent spends caring for the children. Pretty vague, and as I mentioned, I don't know my exP's actual earnings. I can possibly make some vague estimation from old bank statements but nothing definite.

Any advice welcomed on what the principles should be here, and how others have figured out their maintenance payments.
What do exPs normally pay towards housing/accommodation (if anything?)
Or is a lump sum usually decided which contributes to housing, bills, food, clothing the kids etc.
Are there any norms re. payments? A few friends of mine in the UK receive between 500 and 700 pounds per month (for 1 and 2 children respectively) Is this kind of standard?

Eventually I would like to move back to the UK where I might be entitled to benefits - here I do not get anything at present (no child benefit - as far as I understand it I am not entitled to it - think it is means-tested not universal) but I should get unemployment benefit when my contract ends in October if I do not find any more work.

Sorry this is so long, and thanks a lot for any words of wisdom Mumsnetters can offer!

[HowLongIsTooLong]

Yes, extra, but thanks for the comment. My lawyer has actually represented a few women whose ex-Ps challenged their right to move back to their home country. They won the cases. I really hope it does not come to that, though. May even be that my ex-P moves abroad for work before I do. We'll see.

[LadyGardenersQuestionTime]

Over here the basis for maintenance calculations is a % of the non-resident parent's income after tax x a consideration of how many children. The calculator is here but in your case would be something like 20% of his after-tax income. Which is fine if he has a regular income with tax already taken out, but if he's a freelancer then presumably he doesn't. But that is also on the assumption that the resident parent pays for all her/his own living costs + the children's living costs.

How about you carry on with the 50:50 on the mortgage assuming a 50:50 split of equity if/when you sell up, plus a monthly maintenance payment based on his last year's accounts, which I assume he must have done?

[HowLongIsTooLong]

Thanks Lady for the advice on the UK system as some kind of guide. Will try to get some idea of his earnings as I go through the bank statements and will also ask him directly (though will assume he may not be totally honest, sadly).