Taking 10% of the blame when you had 50% isn’t accountability. It’s a discount.

A partial admission is built to look like honesty while costing less than honesty does. You get credit for owning something and keep most of the ground, and because you went first, nobody wants to argue the percentage upward. The other person usually notices anyway, which is why the goodwill does not arrive.

When you own something, own the size of it as well as the fact of it. Say roughly what share was yours without rounding down. If you are unsure, guess high, which is the only direction in which guessing wrong does no damage.