Very good work, LJ. Your vet is likely constrained by the fact that if, heaven forfend, there was some issue that could be traced to the pergolide, he would look pretty feeble without good data to back up why he raised it when the ACTH was "normal". Dr. Kellon and the list members, through the accumulated data from the many horses on this list, have the advantage of seeing many hundreds of PPID horse cases that clearly do best when the ACTH is teens to low 20's. We also (as you have done) get to see what happens when the ACTH is high normal. There are currently no studies done on this, so your vet pretty much has to go with published data, some of which are...misleading.
If your vet just can't make himself increase the dose, then re-test mid-July. There should be enough of an increase in ACTH to show that an increase in pergolide is warranted. Even if you do get an okay to increase the pergolide, I would still re-test, but you could wait until August.
Jaini Clougher (BSc,BVSc)
Merlin (over the bridge) ,Maggie,Gypsy, Ranger