Should I have reduced his dose so much so fast?
Yes, Charleston has earned 3 dose reductions on the Prozinc SLGS protocol. Anytime you get a BG level < 90 mg/dL, you reduce the dose by 0.25U IMMEDIATELY. Next shot is going to be that reduction. Glad to see that you took all 3 of those reductions. Good job!
He also seems to have a bit of an early nadir, around +3 to +5, so you want to try and test around that time to catch those lows, and bring his BG numbers up with food. Do you know how to do that?
Don't worry too much about the pre-shot tests right now. They are going to be high, as Charleston bounces off those low numbers. The pre-shots are the last to come back to a normal range with Prozinc. So keep that in mind.
Do I need to start testing for ketones (he doesn't have a history of them, but I have ketostix).
Yes, please keep testing for ketones, or start testing if you do not test already. With the fast dose reductions, there is the possibility that ketones could develop. Not enough insulin + not enough food + an infection/inflammation (bad teeth, UTI) can lead to ketone formation which can lead to DKA. And you do not want that to happen, since it's a VERY expensive multiple day stay at a 24 hour vet clinic.
The lows are scary, but Charleston needs to be down in those low blues and even better the high greens, in order to give his pancreas time to heal. This new vial of Prozinc really seems to be making a difference in his BG levels.
Insulin doses are not static, as you are finding out. It was only back on 5/29 that I suggested you increase the dose to 2.5U, and now you are back down to 1.5U. Keep up the good work!!!!