Good post from the publicly accessible archives of another online List about Test Values and ACTH stim tests and Interpretation:

https://tvmdl.tamu.edu/?s=canine+cus...t=Search+Tests
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*Moderator's note:

We mustn't forget that that every dog's case may be a bit different, and although most dogs do very well with ACTH test numbers (cortisol levels) at the lower end of the target range of 1-5 ug/dl, in some cases there may be other factors or conditions which may complicate or alter the treatment goals to some degree for a particular dog, and for some dogs this could mean that they may do better with ACTH test numbers a little closer to higher end of the acceptable target range.

This would be something that should be decided by a knowledgeable Vet who has a good deal of experience treating Cushing's dogs successfully.

Successful management of Canine Cushing's Disease depends very much on a combination of owner diligence, observation and understanding and on having a strong and trusting relationship with our Vets, with all of us having the one goal always, which is to do everything we can together to keep our little ones as well as they can possibly be.