Welcome to the vampire club!!
What insulin is he on? How long after his morning shot was the 452?
452 is high, but that's all you've got so far....maybe. Do you have any sense of how long he's been on the 3U and whether he has had any problem with it?
In the beginning, you will find yourself testing more.
I think if I were facing this (and I've got some experience, whereas you are brand new), I would try to shake up his world as little as possible at the beginning. The meter is going to be your best friend. I would keep feeding him what he had been fed before, in the same amounts and at the same times, and give him the same insulin dosage until I got maybe 6 readings on him over the course of a day. Then I would reevaluate a "do-over," starting him back at 1U or even .5U and see how things change.
I would also be testing his urine for ketones (unless the AlphaTrak tests blood ketones. I don't know whether it does).
I think you've been doing alot of reading, so you have some sense of how insulin, stress, and food work together in the diabetic cat.
The important thing right now is to keep him safe. Not so high that you get ketones; not so low that you get hypo. After he settles in a bit and you start learning his rhythms, you can start seeing how a lower carb food, or a little more or less insulin changes his numbers--and his clinical symptoms.
Hope this helps a bit.