I think you should list all your incomings and outgoings and work it out from there.
For example you may be getting maintenance for your kids etc. Personally if I was contemplating moving in with someone I wouldn't expect them to pay 50% of mortgage/household/food bills etc as he would be joining a family of 4.
My mother and step-father had an arrangement where he paid the mortgage (in both their names) and ran his own car, mum then paid 90% of the utilites, food and ran her own car plus us 3 kids' expenses. She had maintenance from my dad and this was in the 80s/90s when interest rates were GINORMOUS so I expect the mortgage virtually swallowed my step-dad's salary!
If you're buying a property and will be sharing the equity then I think 50/50 on the mortgage is imperative tho because in this day and age I'd want to be able to demonstrate that I contributed to building equity in the property iyswim?
Good luck tho, happy for you that you're ready to live together.
Sarah
