I wanted to add my welcome to Lantus/Lev Land! Bananas is looking pretty good!
As far as the vet goes - one thing we've learned here is that many vets don't have that much experience with the dosing of the long-lasting insulins like Lantus and Levemir. They know that those are good for cats, but the dosing is very different than the older style of insulins. With the older faster-acting insulins, the dose was adjusted based upon the preshot number, then the insulin was working pretty much within minutes after you shot and was out of the body a few hours later.
With the depot insulins, a cat doesn't need to eat at the same time as they are getting their shot. As long as they are likely to eat, ie, not sick, the insulin isn't going to onset (become active) for about 2 hours with Lantus and about 4 hours with Levemir, for the average cat. So you are essentially shooting a number a few hours away.
The other major difference is the depot action - that's pretty important to understand. There is a great description of it in the yellow sticky called "Lantus & Levemir: What is the Insulin Depot?" - look for the link in that on the difference between Lantus and Levemir. You can find that the top section of the post
"Where Can I Find It?" which is basically a table of contents to the info in this insulin support group. The bottom section of that post has links on various topics - you probably don't need those yet.
Lantus dosing is also based upon the low point of the cycle, the nadir, which in some cats is +6, but in many cats is not. The nadir can also move around from one cycle to the next.
If your vet is good otherwise, don't be too hard on them about their lack of understanding of how the dosing works. It's really, really common. As long as you can work with them otherwise, you might want to stay with them. It's not that likely that another vet would know more about dosing. These really are newer insulins and they work differently. So many people euthanize their cats upon diagnosis that i doubt most vets even have the opportunity to learn how the insulins work.
The good news is that you've found FDMB and we do understand it. Many people just get their dosing advice here but continue to see their vet for everything else.
Keep asking questions and it will all get a lot easier as you understand more.