The budget frame
Every scene has a fixed light budget; the three dials divide it. Spending a stop on shutter (motion control) requires earning one back on aperture or ISO. The triangle taught as a budget makes every "which setting?" question concrete: what side effect does this photo need, and what does it cost?
The side effects are the photograph
Motion blur, depth of field, and noise are not flaws to minimize — they are the photographic vocabulary. The silky waterfall IS a slow shutter; the isolated portrait IS a wide aperture; the gritty documentary frame sometimes IS high ISO. The calculator handles the budget so the vocabulary stays deliberate.
Learning the scales
The standard stops (shutter doubling, f-numbers in √2 steps, ISO doubling) are worth memorizing — mental arithmetic at the scene outpaces any app. Until then, the triangle calculator bridges: enter what you know, change what you want, read the compensation.