Waterproof Garments
The tape does not waterproof the seam — the overlap either side of the needle holes does. Everything else is just carrier.
— mm
Sound fabric the tape actually covers beyond the stitch line
Resistance is capped at the tape's own rating — no amount of overlap makes a seam better than the tape across it. The linear relation between overlap and head is a fit over the usable range and says nothing about a tape that has lifted at the edge, which is the actual failure mode; check peel and hydrostatic on aged and laundered seams, not just fresh ones.
Seam Sealing Tape Width, Overlap & Hydrostatic Resistance — free, with the formula and a worked example, at Textile School.