Please don't be so tough on yourself. It can take a while to understand your own cat's SS let alone be able to figure someone else's cat's SS out. You've been using a SS for 3 months. That's not all that long to try to sort things out.
First, nadirs can and do change. My kitty had an early nadir -- usually around +3, except when it wasn't. Rather than think of the nadir as a fixed point, think of it in terms of the average of when you see the low points in the cycle. Lantus is "supposed" to nadir around +6. That's the ideal. Cats can have an early or late nadir but there are lots of factors than can cause the nadir to be at a different time.
Numbers will fluctuate. You have no idea how long your cat was diabetic prior to being diagnosed. Her pancreas is still adjusting to insulin. Also, insulin is a hormone. It doesn't act like an antibiotic. Things like exercise, food, stress, etc. influence the action of a hormone and as a result, numbers will vary. In addition, cats hate to be predictable. Sissy wants to keep you on your toes!
It helps to remember that Lantus is a depot insulin. It's the action of the depot that is likely adding to your feeling confused. Some of the ups and downs are due to bouncing. That's the hormone thing, as well. Basically, Lantus isn't as potent as the shorter acting insulins. So, if your cat is in high numbers, it doesn't do the greatest job of pulling the numbers down. However, if numbers are lower, it will be more efficient at flattening out the cycle so you see those lovely blue or green surfs. It also makes it counterintuitive that the same dose that works to lower a higher number won't cause a lower reshot number to plummet. It doesn't. It may help if. you look at some other people's spreadsheets whose cats are regulate. You'll find more of those Kitties on the Lantus board.
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No, seriously, I feel much more confident than I did. It does still freak me out a bit. But I'm ready to do it.