Slapback is usually considered 75-200 or 250 milliseconds, plus or minus. Start at the lower end of that range with one or both of the delay times. 50% wet in the mix; little or no feedback. Use some kind of tape delay or analog algorithm. Dial up the delay time(s) until it sounds right for the context of your song.
The effect started out with tape decks, and the distance between the record and playback heads at 7.5 IPS and 15 IPS (more rarely 3-3/4 or 30 IPS). If you search those terms, you'll see that the delay times mentioned fall around that range. Chet used several kinds of early tape echo units and Echoplex, so I don't think that you're locked in to studio deck IPS times. The exact figures will vary from machine to machine, anyway.