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Carlton, there is another way to find barlow magnification right at home and without a star. Just with a ruler.
1. Place a ruler somewhere in a distance. Say 6 meters.
2. Use an eyepiece with a narrow FOV so you don't have vignetting with barlow.
3. Place a scope on a mount and make sure that you can see only a part of the ruler without a barlow. And it's in the center of the view like on attached photo.
4. Count millimeters you see (N1).
5. Put barlow.
6. Count millimeters you see again (N2).
7. Divide N1/N2 = magnification factor.
I've found it's much more convenient (and accurate) than using a star.
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