I need help with what should be a simple math equation.

I need to create letters that demonstrate 20/20 vision from a distance of 6 feet. Like a snellen chart that is accurate at 6 feet. Typically these are created to work from 20 feet and I found one from 9 feet but nothing for less space. There is a mathematical equation that is supposed to determine letter size based on distance and that's what I need. [login to view URL] If you follow the link above it seems to explain the math needed. You should be able to make sense of what it says there to give me letters that are correctly 20/20 vision from 6 feet away