Just a reminder that she earned a dose decrease with the 41 tonight. So even though it is hard to measure, you have to reduce the 0.25u. There are photos of the small doses
here that you can look at. A good light, a magnifying glass will both help you. It is important not to give the 0.25u dose again because you now know that it can get her to 41. That test isn't on the ss yet, but it's the most important one tonight. You can always put it in the comments if you can't figure out how to put it in the ss. The ss gets easier as you get more practice!!
You also were doing a lot of carbing her to keep her safe tonight, so that dose decrease is extra important.
As far as morning goes, I would play with your 0.25u dose and see how many drops are in it. Somehow you have to figure out how to reduce the dose. And since she's got the recent history of DKA, skipping isn't a great option. Although high carb food will likely make her happy. The very smallest dose is one that you can "vacuum" up - hold the plunger in tightly to the syringe so hopefully, all the air is out. Put the needle into the insulin and release the plunger. That should "vacuum" up a very small amount. Practice with these, however, so that you are confident about the dose you give in the morning.
A couple of questions you asked about - renal threshold varies from cat to cat. Some folks have found that their cat spills glucose into their urine as low as 150. The only way to know for sure would be to test her urine. . . but honestly, I doubt you need to worry about that. I agree that these lower numbers are because you've changed her diet - kudos to you for getting her switched over!
How long does it take for HC to wear off? Another one of those ECID (every cat is different) things. When punkin had low numbers, I tested until he was clearly rising and past his nadir, and I wanted at least an hour after having given him HC. Take good notes and you'll hopefully be able to figure that out for Lily too.