Shooting low (shooting the full dose into normal numbers) is great for gaining control of blood sugar and flattening things out, but you don't want to start out doing it when you can't be home to monitor. In your case, until you've got a little more data telling you what she's doing with this dose, I'd make a conservative choice in the mornings if the preshot is lower than normal. Chris' suggestion of 0.5u as a token dose is a possibility.
How do you feed her during the day? For those who work during the day, having a way to dispense food to a diabetic cat can be a lifesaver, literally. Many people get autofeeders - I had a PetSafe 5 compartment feeder for punkin. It has 4 covered compartments which means you can set the feeder to feed the cat 4 different times.
Here is a good thread (sticky) on managing a diabetic cat when you work full time. One of the best suggestions I've seen is to get 2 of those feeders, so that if you need to, your feeder could actually open 8 times during the day. Take a look at that link above and see if there are some ideas that would work for you. It's pretty important to have food during the cycle for a cat. If their blood sugar goes low, they really need access to food. Many cats will seek out food, although not all do.
As far as your vet, testing and the spreadsheet goes . . . I'd stop showing it to the vet. Nobody else uses the ss except those of us on FDMB, so the vet's not going to expect it. Is your vet helping you with the dosing adjustments? Wendy's point is really good - you'll find that even something as small as a quarter of a unit adjustment (0.25u) will make a significant difference in your cat's blood sugar.
I think in your case, I'd go ahead and decrease your dose by 0.25u as though you were following the
Start Low Go Slow dosing method. That's what we suggest for people who either are feeding dry food, or who aren't able to test much. If you can always get a test before you go to bed then you could use the Tight Regulation dosing protocol, but if you can only get preshots, for safety, I'd suggest the SLGS method. With it, when the blood sugar drops below 90 you decrease the dose immediately by 0.25u. Dosing decisions are made by how low the dose causes the cat's blood sugar to go - which is why I'm focused on the 89 that you got this evening - so knowing how low a dose can make a cat drop is really important.
For figuring out those lows, you can get spot checks in, like grabbing a test before you go to bed, or you can do a curve once a week. That's described on the SLGS link above. Somehow you want to know how low she's going so you can keep her safe.
Are you familiar with bouncing? There's a description of it in the second post in
this thread. I mention it because it wouldn't be surprising - in fact it is likely - that Stella may bounce from hitting green numbers today. If she does, she may be high for as long as 3 days - til Saturday night - and during that time if she IS high, it doesn't mean that she needs more insulin. The bounce will pass and when she comes down, then you'll be able to see what the 1.75u dose is doing for her.
And I just wanted to add a word of welcome - glad to have you here. It's a nice place to be. Ask as many questions as you have and we'll try to help you figure out how to help care for Stella!
None of us are vets, but we learned from others and then stick around to teach new people. You learn a ton taking care of a diabetic cat 24/7 for years.