How the stash calculation works
Subtract the finishing reserve from available grams. Divide what remains by (1 + allowance ÷ 100) to find the base body budget. Multiply by the swatch area per gram, then divide by your chosen width.
With 700 g available, 40 g reserved, 10% allowance, and a 10 × 10 cm / 5 g swatch, the base body budget is 600 g. At 100 cm wide, that supports approximately 120 cm of length. The reserve is not automatically estimated for you.
Use the same construction as the swatch. This tool does not round to complete rows, predict border use or check individual color supplies. A narrower width allows a longer body for the same area. Try 80 cm wide in this example to get about 150 cm long.