What an interesting series of projects. You've carefully thought it out.
For the stitches per inch, I'm assuming you've tried a gauge swatch to give you the actual number that you'll get. For the star, you could try graphing it out on knitter's graph paper (specifically for knitter's because knit sts tend to be wider than they are tall). This site
may help with printing out the size you need and if not there are others on the internet. There's also a site that may help with the overall design of the afghan here.