Use the full measured swatch and the yarn it actually consumed. Enter the same yarn, hook, stitch and carrying technique you will use for the project.
Measure the swatch you weighed
Use the entire swatch dimensions, excluding loose tails, rather than weighing a large swatch and measuring only its central 10 cm. Block it as you plan to block the finished work.
An example you can check
An 8 g swatch measuring 10 × 10 cm scales to 96 g for a 30 × 40 cm panel. With 10% extra, that is 105.6 g. At 125 m per 50 g ball, it is 264 m and 3 balls of one color.
Planning tapestry colors
Measure each color actually used in a representative colorwork swatch, including carried strands. Stitch-count percentages alone do not account for carrying. Different motifs and carrying distances can change consumption. Buy each color separately; leftover yarn in one color cannot replace another.
What is not included?
This is an area-based estimate for comparable flat fabric. Shaping, bag bases, handles, seams, borders and unusual tension changes need their own allowance or separate calculation. The default 10% is editable, not a guarantee.
Method and further reading: Play Hooky with Me