I did a test with my 2330l...which is similar. it can be really hard to tell exactly.
I'm guessing about 9.5 inches in daylight to get text to focus to start to look clearer. In night mode some of the text isn't in focus, only the bigger stuff. I logged into the cameras f/w to see 720p as mydlink lite only does 480p max.
But then again these cams aren't really for reading text,especially at 1 megapixel.