I am not a vet, and this repsonse is solely based on my past experience
with my ferrets and my vet's advice on same.
 
I've had ferrets that took daily doses as high as 2 mg/Pred and .3ml of
diazoxide (Proglycem) twice a day.  This would be a daily total of 4mg
of Pred and .6ml of diazoxide (Proglycem).  This was in the latest stages
of Insulinoma, when a possibly long-term toxic dose of medication was
preferable to a deadly seizure.  I've had three ferrets that had an
extended quality of life for six months or more on these meds at those
dosages in the past.  My suggestion is to refer your vet to "Essentials
Of Ferrets, A Guide For Practicioners," by Karen Purcell, DVM, published
by AAHA Press.  Let him know there's a formulary with detailed dosage
information on a wide variety of ferret related pharmacueticals, as well
as being the single most comprehensive ferret medicine book available.
Let him know it's a ferret specific update on the original "A Practioners
Guide To Rabbits and Ferrets" by Susan A. Brown, DVM, and it's more
up-to-date with current knowledge than "Biology and Diseases of the
Ferret" by James G. Fox, DVM.  (And no offense meant towards Dr.'s Brown
and Fox, who's books were groundbreaking in ferret medicine.)
 
Good luck!
Anne
[Posted in FML issue 5193]