Finishing
Narrowing the chain and overfeeding both concentrate the same mass. Only the finish adds any.
— g/m2
From the width change, the overfeed and the add-on
The balance assumes mass is conserved apart from the stated finish add-on, so it excludes any weight loss from the drying of residual moisture beyond the finish and any fibre loss from shearing or brushing on the same line. Add-on is dry solids on the weight of the entering fabric; if the recipe is stated as a bath concentration and a wet pickup, convert it first. Width change is treated as uniform across the frame, which ignores the bowing and edge effects a real stenter produces - the delivered width at the centre and at the selvedge differ, and a GSM cut from the centre is not representative. The frame is assumed to hold the set chain width, which a heavily overfed knit will resist.
Stenter Chain Width, Overfeed & Target GSM Solver — free, with the formula and a worked example, at Textile School.