Testing
Slippage scales with cover factor squared. A modest shortfall in force is an immodest change in construction.
— N
The worse of the two directions, which is the one that fails
The exponent is an empirical fit and the single input that decides how alarming the remedy looks; two is a reasonable working value across ordinary apparel wovens and should be re-fitted from a pair of measured constructions where a real decision rests on it. Everything the model says about direction is more reliable than what it says about magnitude. Note which force governs: a seam sewn across the warp is resisted by the weft threads sliding, so the weaker direction is usually the one whose threads are fewer and coarser, and it is the one to fix. The remedy is expressed in picks because picks are the only sett a weaver can change without re-warping, but the arithmetic frequently returns an increase that no loom will insert into that warp - and that answer is the useful one, since it says the fabric needs a different yarn, a different weave with more intersections, or a resin finish rather than a small adjustment. Nothing here models the seam itself: stitch density, seam allowance, needle size and thread all move slippage substantially, and a marginal fabric can be rescued or ruined in the sewing room. A finish that locks the yarns, from a light resin to a mechanical treatment, moves the result outside this relationship entirely.
Seam Slippage against Cloth Cover Factor — free, with the formula and a worked example, at Textile School.