Lorna in Ontario
Considering the details you have given, if it were me I would increase by 1mg right now. I would watch clinical signs,making notes in my daily journal . I'm guessing you should see a positive sign....don't ask me,every horse is different....within several days. Maybe not.
But, for example, a horse's weepy eyes can clear up. Or a slightly different energy(more) level can appear. Or slightly happier horse. Sometimes signs are subtle,sometimes more obvious.A sore horse can appear obviously less sore...if trim is appropriate.
Again,for me ,considering his numbers,it would make semse to go up another mg in the next week.
Repeat careful watching.
I'm not a vet.
This is just from having 7 of my then-herd of 9 dealing with PPID and IR.
Lorna in Eastern Ontario