In the past, when shooting close-ups I found that conventional DOF calculators could be quite a bit off.
So I wrote one based on work by Richard F Lyon, once a scientist at Foveon, Inc.
kronometric.org/phot/iq/DepthOfField-Lyon.pdf
See an example here:
www.dpreview.com/forums/post/68424568
One inputs real size; size on the sensor; sensor size; f/number; marked focal length ... it tells you the DOF and the necessary shooting distance, both useful to know for macro shooting.