Are you using a human meter? If so, 68 is a beautiful nadir, as long as she didn't go lower later in the cycle. If you are using an AlphaTrak, then 68 does signal a dose reduction. The next time you get a number like that (again if you are using a human meter) give her a little snack of regular food and retest in 20 minutes. It's best to save the syrup until she is below 50.
I would get more nadirs whenever you can - day or night. It could be that her nadirs are too low but I don't think today told you that.
Another thing you could try is to skinny up the dose for a yellow preshot (maybe 2 units minus a few drops) but continue with 2 for pink and higher pre shots. It is fine to shoot different doses on different pre shots with ProZinc. Some cats respond well to that; others like consistency. It will depend on how she reacts.
Also, maybe add your meter type to your signature. It makes a big difference for advice given on the lower ranges.