Most new guys simply don't have the training or knowledge of their body to run successful dbol only cycles - actually I'd venture a lot of more experienced guys don't either.
I've run multiple dbol only cycles and retained the majority of my gains - there will be some water loss (generally a few lbs. in the first few days) and the normal loss from going natural again. That's it, as the rest is attributable to user error.
That being said - if you absolutely intend to do it:
I'd start at 20mg daily. Give it at least 7 days. If you aren't seeing progress go to 25 and then 30 if absolutely necessary. I'd be suspect if you aren't gaining on 25mg as there just aren't too many guys with genetics that suck that bad so it would be fake or your training/diet is screwing it up. FYI if you aren't deadlifting (not SLDL) and squating you are missing the boat.
Anyway - run your dose but when running dbol alone I would taper it down before starting clomid. Don't go cold turkey. Some will argue with this and may even sound logical but everyone that I know who's retained gains from dbol-only used a taper and most of the guys who lost it all didn't. Dbol is also the drug for which the taper was invented although it has been grossly misapplied to other compounds.
Something like this. 20,20,20,15,15,15,10,10,10,5,5,5
I've even gone to 5mg EOD for a period of 6 days. Probably overkill with clomid but the point is to not go from 30 or 25 to zero. Bring it down to at least 15 but I'd go lower and play it safe as the crash can be pronounced when droping larger dosages.