I’m a SP of many years, and for a long time I was the higher earner (it’s the other way round now). We’ve always taken a ‘pay proportionately in accordance with earnings and the rest of it is yours to do with as you wish’ approach to rent/mortgage, bills etc. Which means that right now DH has more disposable than me each month, in the past I had more than him. But it’s always worked out that whoever has more spare cash pays more for takeaways, holidays, nights out, clothes etc, so we are both living the same lifestyle, if that makes sense.
I have never directly subsidised DH’s maintenance, and he would never ask me to. I always took the view that before I came along, and if he was still single, he would still have to pay £x, and that if circumstances change (e.g. he was out of work for a while once and then had to take a lower paid job), that’s for him to sort and agree with his ex as regards maintenance, but if, for example, he wanted to keep the maintenance at a higher amount, then he can’t reduce his contribution to our household to facilitate that). I have no issue with paying for food, treats etc for SDC when they are with us, and in the long run over the years I have no doubt ended up paying more towards our bills, and nice stuff for me and DH because I’ve had more disposable income, but I won’t directly subsidise money that is going out of our household into their DM’s.
Thinking about it, if you are going to be the higher earner and the bills in the 'pot', incl CM, are split proportionately, surely you will end up paying more towards the maintenance than he will?! Not a cat’s chance in hell. There is a reason that maintenance via the CMS is calculated on the NRP’s salary only, because they are the only person with the responsibility to pay it.
Now DH earns more it’s much less of an issue – he’s able to meet his agreed share of the bills and pay whatever maintenance/treat the SDC to whatever he likes without it affecting me, but it’s trickier when you have similar incomes or the SP is the higher earner.