A few things in response to some of the comments:
- Please continue testing because as Serryn (who is very experienced) mentioned, a cat that has been that low, or had a symptomatic hypo, could be more sensitive to insulin after the episode.
- If you look on this
"Where Can I Find?" post, which was prepped specifically for use on the Lantus/Lev Insulin Support Group, in the lower part of the first post you'll see 2 links to cats that were accidentally overdosed. You want to be careful for at least a few days AFTER the episode happens. An extra 20 units is huge - I would not relax until several days are past.
- Diabetic cats have the potential to have their pancreas heal and for it to begin sputtering back to life and putting out insulin. If that happens, the dose that was "just right" before can become too much now. Home testing will let you know what you need to do with the dose - but unless a cat is kept in constant relatively high numbers, they will need dose adjustments. I'd highly encourage you to always test before each shot, and if at all possible, to get one test in the middle of the cycle (anywhere from 3-8 or so hours) after each shot. You will soon see that a cat's blood sugar doesn't remain constant. A tightly regulated cat might be constant, but you are not at that phase yet. Not every cat will be able to heal their pancreas, but some can and some go off of insulin because of it. There is a see-sawing quality of adjusting the dose downward if the cat's pancreas does begin to put out some insulin. As the cat's own body increases its amount, the amount injected decreases 0.25u at a time.
- Lantus dosing is based upon how LOW the dose can take the cat. The low point is typically, but not always, somewhere inbetween the shots. So in order to know what to do with the dose, you really need the information from the mid-cycle. What's most helpful is to have a spreadsheet, then sprinkle the tests in here and there. You don't have to test every hour, but if you think of the spreadsheet like a jigsaw puzzle, having only a stripe down the middle tells you far less than if the puzzle pieces are sprinkled around throughout the puzzle.
- I would strongly encourage you to use syringes. You can draw up a sample syringe, as Serryn described, and mark the 2u with masking tape, then always compare to it if you want. You won't make this mistake again - it's the kind a person only makes once, LOL. What we know with cats is that they respond to very tiny changes in the dose - you can change a cat's blood sugar even by "fattening" or "skinnying" a dose, not even a 0.25u increment. That's why the ability to accurately adjust the dose by less than 1.0u is critical. The variations that Louellen is talking about are likely because the syringe markings are inconsistent. We've found that to be true, so many people either use a sample syringe to compare for dosing, or use calipers. With both of those techniques, you ignore the syringe markings. If you compare 10 of your syringes, the markings will no doubt not all be same - looking at the distance down from the hub. On that same post I linked above, if you look for "Dosing with Calipers" you'll find instructions on how to do that. We don't usually suggest that to new people because it's overwhelming enough just starting, much less worrying about syringe markings, but since the topic has come up, I wanted to give you the info.
There is a lot to learn but fortunately, you've hit the jackpot with diabetic cat websites. There are many people here who can help you learn what you need. I'd encourage you to begin posting on the
Lantus/Levemir insulin support group where you will find all Lantus/Lev users who can help you learn these finer points of how to take care of your sweet Chicken. You will learn a ton by reading other people's posts there as well.
So glad she's doing ok now. Hang in there. The beginning is overwhelming, but as you learn more, it all gets infinitely easier.
I know how incredibly stressful yesterday must have been for you and I'm so happy that you found this site and were able to get the help you needed to keep Chicken safe. 
