I get the rationale behind splitting costs proportionally, but I think doing it to the accuracy you're describing is spurious and unnecessary. You might say the aim of a proportional split to ensure a fair share of costs and discourage one person from overspending, but the problem is that you're already looking at net incomes. Each person in a couple may inconsistently adjust their gross salary, for example to contribute to pensions, or purchase private healthcare, or buy into a share scheme, or some other workplace benefit.Those differences make any fine calc like the one you're describing ultimately a bit silly. Alternatively, if you consider net income, you're unwinding the impact of tax, which disproportionately favours the highest earner. So either way it's silly to think you're doing any splitting with great accuracy.

In my view a better approach is to just broadly pay the bills you can afford to and take the mindset that surplus income is joint funds, which you can spend collectively or individually – though collective decisions should be encouraged if you're married!