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Why does it have to be static in the first place? Why not just let them contribute what they can, when they can, since the money’s not tight?
Who is to decide when and what they can pay then?
It’s also as much about determining the disposable income. If she has a different opinion on what is reasonable to spend on other things that could easily become a can of worms.
“This is what you need to contribute to the household, whatever you do with the rest of your money is not my issue” is much better than: “Hey, I know you’re low on cash but maybe if you cut back on lattes, avocado toast, gambling, booze and cigarettes, we would be able to pay the bills.”
In reality, the fixed amount isn’t very fixed anyway. If one part can’t pay, it’s still unlikely that the partner would kick them out. But as long as money isn’t that tight, it’s simply better to allocate a fixed amount to the household, so the money isn’t disposable for random spending, so they don’t risk overspending or increasing expensive habits.
This isn’t just to curb the costs, but also to avoid the situation in which one part becomes financially dependent on the other, which is also a recipe for disaster for both parts.
Math is off.
He makes 30% more than her. If she makes 100, he makes 130. The total income is 230.
Her income is 43% of that (100/230) and his is 57% (130/230).