FWIW, splitting up finances does not have to be an irrevokable step towards The Big D. W and I split up our finances back when she was still "heading out the door" - but I approached it as a very positive thing. Even now, the finances are split - and I continue to be glad we did.
Here's how we did it, FWIW. We have three accounts: hers, mine, and family.
Each of us automatically deposits our paycheck into our individual account. Then, we each automatically transfer a set amount per paycheck from there into the joint account. In our case, that amount was calculated as a percentage (not strictly 50-50) after accounting for how much each of us earned, what were were getting in retirement benefits, etc. The goal was to be as fair as possible in determining that amount.
All household- and kid-related expenses are paid out of the joint account. I did (like you did, I think) a thorough assessment of what our monthly bills were, and we went over them together. We did make some adjustments based on her MLC frame of mind, but for the most part, groceries, utilities, and the like didn't vary much based on her plan to leave.
We agreed to review and adjust the amounts we were contributing at least once a year, or when anything significant happened to change things - whether that was her moving out, or one of us getting a raise, or whatever.
By agreeing on the automatic, scheduled transfer of funds, we took the "nag factor" out of the equation.
The two keys for me, I think, were:
1. Approaching the whole thing with a spirit of wanting to be fair and reasonable.
2. Acknowledging that she planned to leave, and putting in a way to deal with that when it came - without having to deal with it RIGHT THEN. Taking that huge looming issue out of the equation was HUGE.
Hope this helps. Even today, I find it very liberating to have "my own money" seperate from the household money.
Thread #10 22 year M, MLC, Piecing since 1/07 Goal: Live with confidence & enthusiasm!