When one of you has their hours cut
Reopen the split the month the hours change, not the month it starts hurting. A cut in hours is a permanent change to what one person can afford, and the reason it is so often left alone is precisely that it is survivable: nobody calls a meeting about a twenty per cent drop the way they would about a redundancy, so the split quietly stops matching the incomes and stays that way for a year.
The cost of waiting is not financial. It is that by the time somebody raises it, they have been quietly stretched for months, and the conversation arrives with resentment already attached to it.
Say the number, not the feeling
"Things are a bit tight" invites reassurance, which is not what is needed. "I'm down about four hundred a month from March" is a fact both of you can work with, and it makes the next sentence a practical one rather than an emotional one.
It also removes the worst dynamic in this conversation, which is one person having to argue that their difficulty is real.
Change the split, or change the outgoings — decide which
There are only two levers. Either the shares move so the person with less income pays a smaller proportion, or the household spends less so both shares fall. Most households need a bit of both, and the mistake is doing neither while hoping the hours come back.
If you move the shares, moving to a proportional split is the version that needs no argument: each person pays the same percentage of what they earn, so the change is arithmetic rather than a favour.
Make it explicitly temporary, with a date to revisit
"Until June, then we look again" is much easier to agree to than an open-ended change, for both people. The one who pays less is not accepting a permanent status, and the one who pays more is not signing up to an indefinite arrangement. Put the date somewhere you will both actually see it.
If the hours go to zero, it is a different conversation
A cut is a recalculation. A job loss is a household with one income and a runway, and the first decisions are about which fixed costs can be stopped rather than about how to divide them.
Easier with the numbers on the table
Money talk goes better when nobody is guessing.
Pairly puts what leaves, what is left and who has covered more in one place both of you can open. It never grades anybody's spending, and it never calls a purchase a mistake.