I quit trying to explain anything about FD to my vets - yes, plural because we have a vet in Michigan and one here in Florida...
It did take me a little while, but after various discussions with vets about feline diabetes I discovered that none of them knew as much as the people on this forum did. The years of experience here outweigh anything that vets have learned while they were in school. And unless they've spent all of their free time learning and reading the literature that's written about FD they just can't possibly keep up with what the best treatment is for kitties with FD. I don't think it takes away from your vet's ability to treat other things, but they just don't know what they don't know!
Max had ONE good day on 2.5 and then didn't...it's possible that her pancreas sparked to life for that day...but then grew tired and went back to where ever they go when they're not functioning well. It takes time for that pancreas to rebuild. And it takes as much insulin as it takes...increasing until you see a steady flow of numbers under 100 is the objective. If you don't see them you'll need to increase and possibly decrease when her pancreas decides to participate...and then increase again until she's able to handle it all on her own...
It's one reason why it's called the sugar dance...we go up/down, up/down and up/down on the dose until that pancreas is fully functional and they don't need any more help from us...
I'm glad your vet was nice about it all...but trying to teach her everything you've learned about FD...well that could take hours.
Perhaps you could share this article with your vet:
http://www.rednacionaldeveterinarias.com.uy/articulos/endocrino/intensive_glucose_control_glargine_cats.pdf?fbclid=IwAR2pfqQsf02W1Te3X8DkPw8DH5JZE-gHwHI16ISGvjOus0lGURGpp5ibXt4
Best to you