I think the 10 foot rule simply means that if you are going to add something new to an already established garden, you want it to be something that will stand out from 10 ft. away. But then I may be completely wrong.
I've done a couple of talks about hostas to garden clubs. The second one that I did is posted on my family web space as a pdf. (The original presentation was a Power Point file). The pdf version, of course, has none of the off-the-cuff remarks, but a lot of the text is incorporated into the slides and there's quite a large section on planning the garden.
If you want to look at it, you'll find it at: clients.teksavvy.com/~frederking/Happy_Healthy_Hostas_2015.pdf
It's a very large file and will take awhile to open. "Planning the Hosta Garden" starts on slide (or page) 79.