How good are your eyes?
I'm just being silly, but seriously the "readable" distance is highly dependent on the viewer and conditions.
The best way to judge it would be to print out a 1:1 image of the candidate display and tape it to the wall where a typical user would be viewing it.
I have attached a 1:1 image of the display as a PDF. If you print that out using Acrobat with "Scaling" set to "None' in the print dialog, it should come out as actual size. You will probably have to tell Acrobat to "tile" the pages, then tape them together since it is too big for normal office paper.
Then just put that at distance and ask your friends and family if they can read it