Having been in a similar position (with different materials) what I did was found a developer/film combo of something that I could compare with the film/developer combo that I wanted to use then made a semi-informed guess.

So, say film A in developer X needs 10mins and my film (B) needs 11mins in that developer I checked film A's time in the developer I didn't have a time for Film B and guestimated based on that. In this hypothetical, say it was 8mins, then I tried 10% (10->11mins difference in devl X) so I'd use 8:45.

Also, DD-X can be use 1:9 instead of the 1:4 recommended on the bottle. Was some discussion in p.net and rec.darkroom (one of them) newsgroup about how much to extend times for that dilution. DD-X is very expensive in my part of the world so I was experimenting with this but have not gone back to it once I finished my bottle.