Size the idea around the invalidation
Choose an invalidation price before choosing position size. In the calculator, risk dollars equal paper equity multiplied by the risk percentage. Quantity is risk dollars divided by stop distance plus estimated entry and stop-exit fees per unit. A tight stop can imply a surprisingly large position.
Put it into practice
Check implied account exposure. Allow for slippage, funding, gaps, and liquidity; an actual stop may execute at a worse price than planned.