The formula
Estimated metres = remaining grams ÷ label grams × label metres. For 30g remaining from a 100g / 200m ball, the estimate is 30 ÷ 100 × 200 = 60m.
How precise is this?
This assumes the remaining yarn has the same length-to-weight ratio as the label. Scale resolution, moisture and differences within a ball can affect the estimate. It cannot identify an unlabelled yarn. Check that the stash matches the exact yarn and shade you intend to use.
Use your result
The planner subtracts your stash before rounding the remaining requirement up to whole balls or packs. The displayed estimate is rounded; the transferred value is rounded down to the nearest 0.001m to avoid increasing the estimate through display rounding.