Hi all, been out of the game for about 9 months so this might be out of date - but I will give my thoughts because I went through the exact same thing.
Firstly I think its fair and just that you made sure all the bills were paid immediately after you left because most people will take some time absorbing everything before they apply for all benefits.
However, once those benefits are in place I really think you should take a good look at what you are paying. Some of the advice has been said before so apologies for repeating but you might want to do something like the following.
Firstly you can as mentioned use various on-line calculators to determine all the benefits your ex will be receiving - and it will likely surprise you. Child benefit, working tax credits, child credits and possibly council tax benefit and even free school meals if your ex applies for them. You don''t necessarily need to ask her, you can determine this for yourself.
You can also use on line calculators to determine what your
child maintenance payments should be - which is essentially X% of your monthly income with a ratio applied based on how often you have the kids overnight. Again, it might surprise you what this is - mine is about 15% of my monthly income.
It was mentioned earlier but it is imperative that you start paying this amount separately by standing order and that the standing order is clearly marked "Child Maintenance". Otherwise your ex could feasibly ask for back payments.
Current benefits do not take into account Spousal Maintenance however as I understand it Universal Credit will - Spousal maintenance will be deducted £ for £ from Universal Credit payments meaning that it is largely useless to an ex who has a large benefit income as and when they switch to UC.
OK - so with that said - I would suggest the following:
Determine her likely total income (including benefits from on line calculator) without anything from you.
Add on your Child maintenance projected payments.
Calculate all her essential bills including obviously quality of life for the kids (clothes, holidays, decent food etc). Take into account reductions in outgoings (you are no longer eating there and neither are the kids x nights a week, her council tax will reduce by at least 25% and possibly water/gas/electric)
Take total outgoings from total income including Child maintenance which you must pay anyway.
Assuming there is still a shortfall - I would suggest that if you can afford it you should cover this, but bear in mind the comment about Universal Credit. This will become Spousal Maintenance and she will eventually lose it.
Once you get past the Essentials and have all bills covered, its then a judgement call. You need to look at how much you need to live on. Whatever is left (hopefully) is then "spending money" and you have to make a call about how much each of you should have of that spending money.
Of course anything over and above the Child Maintenance will essentially be an informal agreement but if you have been fair and have really taken into account all her outgoings you hopefully wont get too much of a shock come the final settlement.
But as Fiona said, the goal of the court is NOT to preserve the same lifestyle of the parent with Care at the expense of the non resident parent. The goal is to make sure the kids are OK and then to be fair to both parties - and that does not mean the parent with the lower income is automatically entitled to parity with the higher earner.
My advice is to be fair - to both of you, and not to be driven by guilt and overcompensate, as I did for a while. It will do you no favours in the long run.
And again - be wary of that Spousal Maintenance/Universal Credit gotcha. Your politics may make you feel this is right and just but eventually anything you pay above child maintenance will likely be lost. This may mean that she cannot afford to live in the FMH if her total income + child maintenance + benefits do not cover her outgoings. But I would seek advice on this, don''t take my word for it.
Hope this helps - as I said been out of things for a while so some of this might be out of date. Please seek advice on the points I''ve raised as I am not an expert.