On my balance sheet (excel) I have liabilities representing future random expenses. Kind of like paying into a savings account, except with an actual extra account.
I generally consider cash minus my accrued liabilities to be the amount of money I "have" at any given moment. When I have an unexpected expense, I subtract that from the liability at the same time the money leaves my account, so there's no affect on the overall balance.
The amount of the liabilities is automatically calculated by formulas in excel, so I don't have to do anything to keep it updated. It even increases the savings rate over time for inflation.
I have separate liabilities recorded for different categories, like new car, car repair, home repair, etc.
Right now I add about $750/mo for all categories combined, so about the same as the previous poster.