I found a formula on a blog today to figure out the number of stitches per inch when your machine only shows you the stitch LENGTH (1.0, 2.5, etc.). Those length numbers are in millimeters. There are 25.4 mm in an inch. If you want 10 stitches to the inch, divide 25.4 by 10 = 2.5!
15 stitches to the inch = 1.7 in millimeters, 3.0mm setting on your machine is about 8.5 stitches to the inch. I use 2.5 for piecing, 3.0 for quilting, 2.0 for paper piecing. So I am using 10 stitches to the inch for piecing, 8.5 for quilting (I like the look of the larger stitch), and about 13 stitches to the inch for paper piecing.