Flying Needle advises 4.5. I prefer 4.6 - 4.8 for most of my flower designs that I stitched on cotton. As you noticed yourself with your moon: puckering is less when you stitch on felt. That does not mean you have to use a different tension for felt, you might be better of adjusting your underlay. Embird let you do that not by changing the density but by adjusting the distance between the lines - see picture. It won't make the design too dense for cotton and it won't make it too light for materials like felt.
Just to complicate matters: the thread you use may make a difference as well. All 40 wt threads are not equal; if one is twined more than the other it would feel a bit thicker and the embroidery might look a bit thicker. I have thread of different brands and one of them is a bit thinner than the others.
When selling designs go for average: most designs are stitched on cotton, use a density that suits that, a nice underlay will help for universal use and most threads are more or less the same.
What you can also see in the picture is that I made a general underlay and that the 2 halves of the leaf have different stitch directions and are overlapping slightly, which works well when you use the same colour.
Thank you. Exactly what I wanted to know. I wanted to make sure I use the density to cater for the majority of people. Thank you for the picture with the settings.
5.0 is the default setting, I often use 3.0 when I have my density set at 4.8. But test out some settings - make 4 or 6 small squares in a 100x100 mm hoop with different combinations and see what you like best.