Here's the setting on my DD-3, but keep in mind mine has been modded by Robert Keeley:
Left to right; E Level at 12 o'clock Feedback at about 1130 Delay time at about 230(I estimate this to be at about 125-130ms) Mode knob set on Medium(200ms)
This makes for a good slap for me. Then I can switch over to the Long mode for slower more "atmospheric" songs. Again, mine is Keeley-modded for analog warmth so settings on a regular DD will probably vary. The important trick IMO to get a rockabilly slapback from a DD-3 is setting the mode on Medium(200ms), then setting the delay time knob at about 230. This gives you between 125-130 or so milliseconds...sorry I'm repeating myself here...then adjust the effect level and feedback knobs to taste.