Sometimes it can take a few tries to get the Pergolide adjusted for a horse like Mable; who sounds like she really does need it.
Here's a couple links that might help you.
Dr. Kellon's website with lots of great information on IR, EMS, Cushings. Equine Cushing's and Insulin Resistance Information
This is Dr. Kellon's Yahoo Equine Cushings Group. It's free to join, ask questions, and research. EquineCushings : Equine Cushings and Insulin Resistance
Any horse with metabolic issues is also managed with a very strict diet. That means absolutely no grain and no treats with sugar in them, including carrots.
Hay should be testd for WSC value but that's hard to do if one can't buy their hay by the season
I have a horse with insulin resistance and another one with Equine Metabolic Syndrome. These two diseases typically don't require drugs, just a grazing muzzle or dry lot and strict diet.
My friend had her Paso Fino on Pergolide the last 18 months and it did help; I can't remember the dosage but it was a lot more than your Mable was prescribed.
Her horse had both cushings/IR, ended up having abscess on top of abscess in his hooves and she's pretty sure had developed pneumonia judging by what the vet had said. He was dealing with too many things that couldn't be helped as his immune system was so weak, so she sent him on to his ancestors in January.
I'm not trying to put a black cloud over you, I just want you to be aware of what MIGHT happen and to not brush anything off, no matter how many times you might have to call the vet.
Hugs to you and Mable who must be just adorable to have the name Mable
I'm old enough to remember the old Carlings Brewery beer commericial "Hey Mabel! Black Label!" That was my first thought when I read Mabel's name