From sewing for dummies:
A setting of 1.0 per mm is about 24 stitches per inch.
A setting of 2.0 per mm is about 13 stitches per inch.
3.0 is 9/inch
4.0 is 6/inch
5.0 is 5/inch
6.0 is 4/inch
They also say: The average stitch length for mid-weight fabrics is 2.5 to 3 mm/10 to 12 spi. The average stitch length for fine fabrics is 2 mm/13 to 20 spi. For heavier fabrics, basting, or topstitching, use 4 to 5 mm/5 to 6 spi.
Hope that helps.