Metrology
A blend has no single regain. Bill it at one fibre's figure and somebody is paying for water — or giving it away.
— kg
Oven-dry mass restored to the blend's official regain
Fibre B's share is taken as the balance of fibre A, so the two always total 100%. Blend ratio itself should come from a quantitative chemical analysis, not the spec sheet — a ratio that is off by two points moves the invoice weight more than a careful regain measurement ever will.
Blended Yarn Commercial & Invoice Weight Calculator — free, with the formula and a worked example, at Textile School.