For business owners, estimated taxes are rarely a flat, once-a-quarter math problem. Between S-corp reasonable compensation, Schedule K-1 timing, seasonal revenue swings, and California's own overlay of rules, a generic “pay 25% each quarter” approach almost always gets it wrong — either tying up cash you need for the business or quietly building toward an underpayment penalty. This week, we're walking through how to build an actual estimated tax model instead of a guess.