I think the 3 units was too much, seeing how he does on 1.5 units, so 5 .... well, I can't even say what I think about that advice. I think you casn also give him 1.5 in the evening, seeing his level (and he was black yesterday). But you have to feel co0mfortable with it.
As far as I can see, he's having a normal caninsulin curve with a second peak, which keeps the level down for a prolonged period, which is great.
For the moment keep him on 1.5 as it can take up to two weeks for the influence of the new food to become completely clear.
When it's clear that he really is too high during his low point, increase with 0.25 and keep that dose for at least 3 days. It also takes some time for a new dose to show its effect: the low point can become lower every day with the same new dose. When he's still relatively high on that point, an increase after 3 days can be done. But if he is near the good numbers, wait 5 days before increasing. Just give the insulin some time to show what it can do. When increasing too soon when the cat is near a good number, you could get him into trouble.
Unfortunately, you can not say: he gets 1.5 unit and drops 300 points, so if he gets 2 units he will drop 400 points. That would be really easy, but it doesn't work that way. The influence of 0.25 unit extra can be enormous or tiny. And the first 0.25 extra can have a tiny influence, the next 0.25 can have a big influence. So always be careful.
What we've seen in ProZinc, is that if a dose is too high, the glucose will continue dropping - have you seen that with Vetsulin/Caninsulin?
What we often see with an overdose is a very fast drop and a very fast rise of the levels if the cat produces stress hormones to counteract the insulin with glucose dumping. In fact, they prevent a hypo this way or turn the hypo: survival mode. They will stay in survival mode for at least 3 days, often longer. Not all cats go into that survival mode: the levels will drop to hypo and beyond ..... It will happen fast. If the low point is normally on +4, it will happen before that moment (how soon is depending on how much overdose it was).
The numbers in the stress period can go anywhere, from very high to very low. So it's necessary to test pre shot levels, because the level could be 'healthy' at that moment and you don't want to give insulin to a cat with a healthy glucose level. But if the cat is very high preshot after a hypo, even higher than he usually was, the dose of caninsulin still must be lowered by at least 1/3. The body is oversensitive to insulin for a period of time.