This user's math is actually closer to correct than they realize, the problem isn't their formula, it's the frame they're using. Dividing a single month's contribution by the total home price will never produce a stable ownership percentage, because ownership in a shared mortgage isn't calculated from one payment. It's calculated from cumulative investment over time.
The real insight here is that fair ownership percentages require a running total, not a snapshot. If Person A pays $1,000 in month one and Person B pays $1,400, their ownership after that single month is $1,000 divided by $2,400 (about 41.7%) and $1,400 divided by $2,400 (about 58.3%). That adds up to 100%. The mistake was using the home's purchase price as the denominator instead of the total contributed so far. The house may be worth $200,000, but after one month only $2,400 of equity has been built, and equity, not the full property value, is what the contributions are buying.
What makes this tricky is the fluctuation. When payments vary month to month, the simple fix is a cumulative ratio: track each person's total paid to date, then divide by the grand total paid by everyone. That gives a living percentage that adjusts automatically. As long as both parties agree to recalculate periodically, say quarterly or annually, the formula stays fair and transparent. No spreadsheet wizardry required, just a running sum and a division.
For anyone navigating shared ownership with uneven contributions, the lesson is practical: stop anchoring to the purchase price. Anchor to what has actually been paid. That shift turns confusion into clarity and makes the math work equally well for couples, roommates, or investment partners. The tool you need isn't a more complex formula, it's the right denominator.