We tend to read print at different distances anyway... eg driving...being creatures of habit a comfortable distance would remain 18 inches. There are occasion where there are specific physical limitations but the individual is the best judge of that... thats why it pays to experiment with glasses on and off.
Aim for comfort.. turning pages etc. and then see if your comprehension improves or worsens. It also pays to consider if you intend to superread and dip if guided to do so by your intuition.
Alex
