I use and have been using 3mm spacing since my very beginnings because I believe in the Proof-Of-Work. I also believe more work = more refinement. I like the classy look.
Most people use either 3.85mm or 4mm.
SPI refers to the number of stitches within one inch. The spacing between pricking iron teeth determines this:
- 4mm spacing → 6.35 SPI
- 3.85mm spacing → 6.6 SPI
- 3mm spacing → 8.47 SPI