Good thought.
I would say 18-20 and would not go any higher than 20.
I would say to keep it separate, as dex may not be what you need, but Solu-Delta or Solu-Medrol.
I would use dex for, say, a pressure biter ... where the dog is severely swollen, but still "with it" mentally ... but would use Solu-Delta if the dog was totally exhausted, hypo-volemic, shocky, etc.
Jack