I spent the week wincing at Bitcoin and a Microsoft buy I'd made too early — while the real money left the house quietly, by direct debit. Seventeen years in, I'd been optimising the wrong end. This week I sat down and worked out my household burn rate: the single number that quietly decides how much I've actually got to invest. Here's what I found, the two levers that move it, and why keeping more of your powder dry matters more than your next trade.