Here’s a simplified calculator:
http://www.dofmaster.com/dofjs.html
which also gives by far the best collected illustrations and explanations I’ve seen of both Depth of Field and the mysterious (but even better) Hyperfocal Distance concept -
“… the hyperfocal distance setting … is simply a fancy term that means the distance setting at any aperture that produces the greatest depth of field.”
“If you set the camera’s focus to the hyperfocal distance, your depth of field will extend from half of the hyperfocal distance to infinity—a much deeper depth of field.”
In other words, find out what the Hyperfocal Distance is for your intended sensor size/zoom setting/f-setting combination (in my case 1.75m), disable autofocus, focus manually at that distance, leave it at that, shoot away, and everything from 0.87m to infinity (in my case) will be ‘reasonably’ in focus.
Apparently you can’t get a better spread than that. But if you want to get closer than 0.87m (in my case), you can, but at cost of less spread. e.g. focusing at 1.25m gives DoF from 0.73 to 4.35m (in my case), which may be plenty, in an interior say.
My remaining question is ‘how in-focus is ‘reasonably’ - and is this good enough for RC?’. It gets complicated! http://www.normankoren.com/Tutorials/MTF6.html