Data from Ohio Schools, Compiled by Dr. J. Raphaelson
Diopters    to     Inches

0 diopters = 240 inches
-1 diopters = 40 inches
-2 diopters = 20 inches
-3 diopters = 13 inches
-4 diopters = 10 inches
-6 diopters = 6.6 inches
-8 diopters =  5 inches
-10 diopters = 4 inches
-13 diopters = 3 inches
READING HABITS OF SCHOOL CHILDREN -- GRADES 1 TO 8
Child's Habitual Reading Distance

Distance  -     #Starting   #Finish

12 to 10 inches  - 287  -    2 **
10 to  6 inches   - 205  -   76
6 to 3 inches      -   11  - 331
3 inch or less      -    0  -   94

** 2 / 503 = 00.398 %
Dr. Raphaelson's statement:

"It can fairly be assumed that what I have found in Clermount County will also be found in nearly all schools in the U.S.A."

For the above data, the starting reading average was -3.7 diopters (or 11 inches), with a sigma of 0.9 diopters, and a final reading position of -9.6 diopters (4.2 inches) with a Sigma of 3.3 diopters.
1