This is very similar to the advice our notary gave us. We opened a joint account, and we collectively agreed to put 75% of our salaries on that joint account. So every time she or I get paid, 75% of it goes on the joint account and 25% remains on our individual - private - accounts.

Anything we buy or pay with the shared account is considered to be 50% hers and 50% mine, regardless of who actually puts more in the account. The logic behind this is that we both make the exact same "effort" in buying those things. So we pay for almost everything with that account: holidays, groceries, utilities, furnitures, ... . If we have extra, we put a bit in a savings account / investment and those also belong to us 50/50.

The benefit of this approach is the it protects both of us. If we decide to have children and that she should work part-time instead of full-time to raise them, she will still have the exact same 50% claim on everything. She will not loose more than I do. We'll just have less money coming in the joint account and will have to adapt our budget accordingly. This is thus a common decision that impacts the both of us equally.

Importantly, our mortgage/rent is also paid off the joint account, so the house belongs to each of us 50-50. I was however able to contribute to a bigger portion of the downpayment for the house. To make things fair, she agreed to sign an "acknowledgement of debt", recognising that she "owes me" half the difference between my contribution to the downpayment and hers. If we stay together, that letter is just a useless paper that sits on a notary desks and we'll never talk about it. If we split, the house will be sold, each of us will get 50% of the proceeds, but I will be able to claim that difference, making things fair.

The remaining 25% on each other private accounts can be used for private expenses. I never have to "justify" buying a video game or an impulsive luxury since it comes out of my private account, I can also go out with friends and not have someone that asks wether that $75 tab at the bar is justified or not. Anything we buy with the private accounts is considered to be 100% belonging to the person who bought it. This account is also used for surprise gifts (birthday, Christmas, ...) since the expenses do not appear in the shared account logs. We agreed that any inheritance will not be split and will remain in our private accounts, so there is no "cheating" there. Also, anything that we had "before" we setup this system remains in our private accounts.

If one of our private accounts becomes "big" and we start seeing some significant asymmetry between those (Eg: I inherent millions and she doesn't, so I could have an amazing life but the join account is frustratingly poor) we have the option of treating some part as regular "income" our of it (Eg: do an automatic payment of $xxx every month). Since this is an income, that amount goes 75% in the joint account and 25% remains in the private account. This increases the amount available on the joint account, so we can afford the cool things we wanted. But the bulk of the money stays in my private account so if we split she won't have any claim to any of it.

Of course, the 75% - 25% split is a decision we took, but you might wanna change percentages according to your situation. Overall we are extremely happy with this arrangement as it brings the financial discussions close to 0 (we had some before).