What’s a fair split in our blended family

Hi, we are blended family of 9 me (m49) my children (son 17) (daughter 15) I have them 50% my partner (f47) her kids 5 of them (daughter 24 - she pays rent and shares a room with her sister) twin girl (18) twin boy (18) son (15) son (8) we all live in a 7 bedroom house which has a granny flat and her older kids live in there and the rest of us live in 5 bedroom house. The oldest daughter who pays rent obviously lives full time and the others currently live here about 70% and the rest with their father.

I earn a lot more than her, probably more than double but her ex husband pays maintenance for the the other 4 who are still in school (twins finish this year). I split the electrical/gas/internet 50/50. I pay more of the rent as I earn more, $750 a week compared to her $600.

In terms of groceries it’s tough. And this is where we seem to struggle. Her kids are at home more and eat a lot more than mine. When it was just me and my 2 kids it was easy to manage as I bought my kids enough food for the week and when it was gone it was gone. We have tried seperate draws with food but ultimately every kid just raids the cupboards and eat each others. My grocery bill has gone up massively and whenever we discuss about working out a percentage of kids and days and putting that money aside to pay for things it creates an argument about money.

Personally I don’t want to have to pay for more than my fair share which I have no idea how we work that out with me being a higher wage earner, less kids and less time. Any suggestions for other blended families how they manage finances.

We don’t want a joint bank account and combine money.