The saddle stitch: spacing and thread.
SPI sets the spacing, the spacing sets the thread, the thickness sets the length — the irons' real sizes from the app's tables.
Stitches per inch → spacing
Spacing in mm = 25.4 ÷ SPI — and the irons come in real sizes: the 7 SPI pricking iron = 3.63 mm, 8 = 3.175, 6 = 4.23 (the maker's own listings). Small goods sit around 3 mm; bags and belts 4 to 6 mm.
The thread picks the number, the spacing picks the thread
The thread's diameter ≈ spacing ÷ 6, plus or minus 10% (the app's fitted rule of thumb, shown as such). 2.7 mm spacing pairs with a ~0.45 mm thread; 3.85 with ~0.65.
Leather weight: oz to mm
1 oz = 1/64 in ≈ 0.4 mm, sold as ranges: a 4–5 oz shoulder = 1.6–2.0 mm (the charts at the big suppliers). The app uses the range's middle per layer.
How much thread — the formula that never runs short
Per inch of seam: 2 + 2 × thickness × SPI inches of thread — the member formula the app checks (1/4 in thick at 5 SPI = 4.5 in of thread per inch of seam). Then add the backstitches, the working allowance, and the tail. The forum rule lands near 4× the seam for two layers of 2 mm — and grows with thickness, the same way the formula does.
The calculator runs this live. Cut generous: the second regret is always cheaper than the first.