Colour Management
Divide by the assay, do not add a percentage. A 94 percent lot on a 100 percent recipe is short in every batch.
— % owf
Recipe dose divided by the assay ratio
Strength correction is linear in the assay, which is a good description over the narrow band a commercial lot varies across and a poor one over a wide range, because depth is not linear in concentration - the Kubelka-Munk relation between concentration and reflectance is, but the visual depth built on it is not, so a correction across a large strength gap should be checked with a lab dip rather than trusted. The assay itself is a comparison against a retained standard, so it is only as good as that standard: a reference sample that has itself aged, absorbed moisture or been decanted repeatedly will drift the whole scale, and the usual symptom is a mill that corrects steadily in one direction over years. Hygroscopic dyes are the common trap, since a powder that has picked up moisture assays low for a reason that has nothing to do with chromophore content and the correction then over-doses; where moisture is suspected the assay should be reported on a dry basis. The stock figures assume the whole drum shares one assay, which is fair for a single lot and wrong for a bin holding several - blending lots without re-assaying is how a store loses track of strength entirely.
Dye Strength Assay, Dosing Correction & Stock Yield — free, with the formula and a worked example, at Textile School.