Almost every separated couple agrees a percentage early. Fifty-fifty, or something weighted to income, usually settled in the same conversation as the schedule and written down in a sentence.
Then it falls apart eighteen months later over a pair of football boots.
The percentage is almost never the problem. The problem is that nobody agreed what the percentage applies to, and two reasonable people can hold completely different views on whether a school ski trip is a shared cost or a nice idea one parent had.
Agree the scope before the split
Sit down once and sort every recurring cost into three buckets.
Automatically shared. No conversation needed, either parent can incur it and the other pays their share. School fees, uniforms, prescribed medication, dental, essential school supplies, agreed extracurriculars.
Shared, but ask first. Above a threshold you both name — $150 is a common landing point — it needs a yes before it becomes a shared cost. Orthodontics, a school trip, a new laptop, a sport that comes with a season fee and travel.
Not shared. Whatever happens in your own house on your own time. Days out, toys, treats, the things you buy because you want to. This bucket is more important than it looks: without it, every purchase becomes negotiable and every parent feels audited.
The threshold matters more than the percentage. It is what stops the arrangement from becoming a running argument about whether each individual item qualified — and it protects the parent who would otherwise get billed for decisions they had no say in.
Then decide the ratio
Fifty-fifty is the right default, and it is the right answer for most families. It is simple, it is obviously even-handed, and it does not require either of you to disclose or argue about income.
There are decent reasons to move off it. A large and durable income gap. One parent carrying substantially more of the day-to-day costs that never appear in the ledger — food, power, the car, the clothes that are just there. A category where the responsibility genuinely is not even.
If you do move off it, be specific about why, and consider doing it per category rather than across the board. Medical at 70/30 with everything else at 50/50 is a far easier arrangement to live with than 70/30 on everything, because it is targeted at a real imbalance rather than functioning as a general statement about who owes whom.
And write down what it applies to. A ratio without a scope is not an agreement, it is a number.
Never make a change retroactive
This is the single rule that prevents the worst arguments.
When the ratio changes, it applies to costs incurred after the change. It does not reach back and re-price things that already happened. Otherwise every renegotiation drags the entire history back onto the table, and neither of you can ever settle anything with confidence, because a future conversation could always reopen it.
Halven enforces this structurally: an expense locks the ratio that was active when it was created, and a new agreement only ever applies going forward. But the principle stands whether you use software or a shared spreadsheet — agree it explicitly, in writing, before you need it.
The corollary is that changing the ratio has to require both of you. A split one parent can change unilaterally is not an agreement either. In Halven a new ratio is a proposal that takes effect when the other parent approves it and never before; a decline leaves the existing agreement untouched.
Getting reimbursed without it becoming the fight
Agreeing the split is the easy half. Most of the friction lives in the settlement, and almost all of it comes from ambiguity about what state a debt is in.
Three habits fix most of it.
Log it when it happens, not at the end of the month. A receipt photographed at the counter is a fact. A cost reconstructed from memory four weeks later is a claim, and it gets treated like one.
Settle on a rhythm, not on a trigger. Pick a cadence — fortnightly, monthly — and clear the balance then. Ad hoc settlement means every payment request arrives as an interruption, and interruptions get read as pressure.
Close the loop explicitly. The most common reimbursement dispute is not refusal to pay. It is one parent believing they paid and the other having no record of it landing. "I marked it paid on the 21st, you confirmed you received it on the 23rd" ends that conversation before it starts. This is why Halven tracks a payment through marked-paid and then confirmed-received, and keeps both timestamps permanently — the confirmation is the part that actually settles things.
Two things worth naming out loud
Child support is not the same as shared expenses. If a support arrangement exists, be explicit about what it is meant to cover, so the same cost is not being claimed twice. Record support payments separately from the shared ledger.
Keep the ledger unemotional. An expense record is not the place to make a point. "Term 3 school fees, $450, receipt attached" is a record. "Term 3 school fees, $450, which I have now paid for the third year running" is an argument with a number in it — and it will be read that way, including by anyone who reads it later.
When it still goes wrong
It will, occasionally. Someone will incur a cost the other thinks was out of scope.
Query it rather than refusing it. Say what specifically is in dispute — the amount, the category, whether it needed agreement first, whether the receipt is missing. A query is answerable. A blank refusal is not, and it converts a single disagreement about one item into a disagreement about the whole arrangement.
Then, once it is resolved, check whether the scope needs updating. Most disputes are not really about the item. They are the arrangement telling you it has a gap in it.