Hello and welcome to this forum. You have a very handsome kitty. Is his 17 lbs his good weight, a little high or a little low? I had a long haired Maine Coon cross too so know they can be a little "fluffier" than other cats.
First observation, other than Jax, everyone commenting here so far has black and white kitty avatars.
Second observation, none of them have had cats that need higher doses, except me. Not saying your cat needs a higher dose, we just don't know yet but it's good to consider it's a possibility.
Some cats, though rare, get 19 units.
I wouldn't call it rare, but rather less common. A cat needs however many units they need. For the record, the cats north of 80 units per shot are what I would call rare, though I've seen a couple in my time here. In all those cases, the cat typically had a secondary endocrine condition causing the diabetes and meant they needed higher doses. Those endocrine conditions are by no means rare, affecting one in four diabetic cats. And actually tend to mean the cat needs on average in the neighbourhood of 7-10 units.
So can I ask, as a less experienced Lantus-user, what the harm is in reducing if we already know the dose was increased inappropriately multiple times? Assuming she'd be checking for ketones wouldn't it be better to drop 2-3 units?
If a cat does need more insulin, you are wasting time getting to a good dose, plus increasing
glucose toxicity. That means the cat gets used to higher numbers and it's harder, needs even more insulin to bring down the blood sugar numbers. If you follow SLGS strictly, reducing 3 units can take 12 weeks to get back to where you were before.
I wrote that he was taken up in dose too quickly and that without testing at key times it’s impossible to figure out what’s going on. Even a +2 or before bed test would help.
I hope everyone one has noticed that the entries in the spreadsheet are not for every day, except the last three days. Before that, entries varied and were several days apart. That does mean the dose was held longer, possibly long enough to build the depot. However, I totally agree on the point on not getting a good picture of what the dose was going before it was increased. Did the vet go entirely by the numbers you provided when deciding to increase? Or were there other tests like fructosamines done? How has his pee volume changed over time? While not as good an indicator, that does say something.
We start with .25 increases.
If a person is following TR, and seeing nadirs over 300, we start with 0.5 unit increases, which is what was done, except after the 278.
Other random observations, not all cats have their lowest point at +6, and the nadir or lowest is not always the same each cycle. Which is a pain when trying to figure out when to test. It can be better to just mix up when you get those tests, just somewhere in the middle is good, maybe +4 to +7. Getting a before bed test is also a good one from a data point of view cause even a +2 can tell us if there is downward movement in that cycle, or whether it's just high and flat.
So a strategy going forward, I think a good place to start is to see what is really happening on 5 units. If it's too high, he'll tell you by showing you lower numbers. With the weekend coming up, would it be possible for you to get 4 tests a day? Two preshot tests, just before the injection, and one sometime in the middleish of the day and one before bed or those midnight ones if you hubby is up. With the data you have so far on this dose, and a couple more days worth as I described, we should get a good picture what is happening at 5 units.
How is testing going now on the ears? I saw early one you were having difficulty with the black ears. I'm sure something almost all of us here can relate to. I put a very fine film of vaseline on the poke site, so the blood would pool there, and not hide in the dark fur. Some people tested on the inside of the ear.
Are you located in the US? If so most of us use the ReliOn Prime meter from Walmart. Costs about $9 for the meter and the strips are $18 for 100 ($0.17 each). 
