Video with the setup and results in detail:
https://youtu.be/fSJHs8ermU0

Graphs for Download:
https://imgur.com/a/sewing-thread-needed-saddle-stitching-qiKTftF

I found using the rules of thumb for thread length a bit unsatisfying, so I ran some tests to figure out how much thread you actually need, depending on leather thickness and stitching iron size. It makes sense that thicker leather uses more thread (since the needle has to travel a longer path through the material), but I wasn’t sure how much of a difference it really makes, especially when you factor in different stitching irons, which also affect the number of stitches per length.

Notes:
When using the graph as reference be aware this does not account for backstitching and excess thread for handling so I usually add around 20cm.

Example:
10cm Stitchline, 2mm Leather, 3 mm Irons.
10cm * 3.1 + 20cm = 51cm Thread

It's also my first non-pattern video and it was super fun to make, so I’d love any feedback or to know if you'd be interested in more content like this.

Cheers,
Josh