Hi Terri and Jaini,
I just wanted to add that you aren't alone Terri - I'm having the exact same argument with my vet about upping Pergolide for the seasonal rise as she is also convinced that the maximum for this time is 100 mg/mL(ACTH from Cornell) and I am really frustrated about the whole thing. I finally got her to write a prescription at least for now so I have been able to raise King's pergolide(ACTH came back @ 49 from Cornell in the middle of Sept). It was a 2-hour argument about it, though! I also have another horse whose ACTH came back at 68 mg/mL and the vet refuses to admit/acknowledge that the horse has Cushings. (I'm pulling my hair out)
Just thought I would offer my support as I know what you're going through. Best of luck!
Stephanie & King
(I'm sorry, I don't have my CH link on my phone right now)