Sewing
A blind hem uses few stitches and a long path per stitch. Costing it on stitch count alone understates the thread every time.
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Including the wastage allowance
Blind hem machines vary widely in how deep the needle penetrates and how far it travels between catches, so a path factor lifted from another machine or another fabric will be wrong. Unpick a measured length of production hem and divide the recovered thread by the seam length to fit it.
Blind Hem Thread Consumption Calculator — free, with the formula and a worked example, at Textile School.