Once we're able to see your spreadsheet, we'll be able to make more informed comments. That said, Novolin (or also called "N" here) is very different than Lantus. N is much faster acting than Lantus. The typical duration of N is about 8 hrs, give or take. Thus, it's expected that you'd see high numbers at the beginning/end of a cycle -- the insulin has been used up.
Lantus has a much longer duration and there is overlap between the 12-hour cycles. You are correct in that it will take up to a week for the insulin depot to form. Once the depot is established, you'll have a better sense of when the lowest point in the cycle (i.e., nadir) falls and what kind of duration your cat is getting with Lantus.
One important factor to keep in mind. With N, dosing is based on the pre-shot number. Because N can hit like a ton of bricks at the time of an injection, you both have to adjust the dose based on what the pre-shot number is telling you and you must have food on board, usually for an hour, before you give an injection. In other words, with N, your cat must eat prior to a shot. With Lantus, the dosing is based on the nadir -- not the pre-shot number. We get the pre-shot test to insure that it's safe to give a shot but it's not what a dose change is based on. One other difference is that Lantus onset isn't immediate. It can take up to 2 hours for Lantus to begin to kick in. Thus, we test, feed, and shoot and most of us shoot when our kitty is busy eating. It's important, though, to have data that tells you when your cat's nadir falls. Some cats, like my Gabby, have an early nadir (versus at the middle of the cycle).
