I have for some time missed the depth of field scale provided on Nikon Manuel focus Lenses. After moving from manuel to Auto focus cameras in 2000. All the autofocus lenses I purchased to replace the old manuel focus lenses all lacked the Dof scale. Although I could preview Dof this was still not adequte as the tendancy is always to use a smaller aperture than absolutly neccesary. Resulting in compromising quality due to excessive diffraction.
The guess work has been taken away by the introduction of stand alone DoF calculators from the likes of Expodisc, but it is now possible to get smart phones Apps at a fraction of the price which will make the calculation for you based on many different focal length and Film/Sensor sizes and even allow you to input your prefrence for the size of Circles of Confusion.
One app which stood out for me is Phocus for the iphone. The app has a clear interface and also provides quick adjustment of preferences and parameters.