Planning a home-repair sinking fund for an older property with unpredictable expenses

Owning an older house means the repair budget is never truly empty. Last month it was a leaking tap; the next expense could be a roof issue. I'm setting up a separate sinking fund rather than treating every repair as an emergency, but a neat monthly amount feels arbitrary. Do you base contributions on an annual percentage of home value, a list of aging components, or simply what your cash flow can support? I know none of those predicts what breaks next.