Better to feed whatever to get the numbers up if you are not sure how she will react.
For future reference, you only need to feed the LC (low carb) when numbers are in the 50's. About 1 teaspoon should do it. If the numbers keep dropping, you would have wanted to feed more LC until you hit the low 40's then start with some MC. A bit of Karo on the gums if below 40.
If it had been earlier in the cycle, around +2 or +3, maybe a bit of MC (medium carb). And monitor.
We want to keep her surfing in those nice greens as long as we can.
You may need to drop the dose of Lantus. Let me look at the SS and get back to you.
ETA: Yes, since only first day on Lantus and depot has not filled yet, I think you want to drop the dose to 0.5U maximum, maybe even 0.25U. Please, get a +2. It can help tell us which direction she may be headed. Early days with the Lantus and the more data you can gather the better. +3, +6, +9 are suggested for the first couple of days on Lantus. Tests in the +4 to +7 range would be good to help pin down her new nadir with the new insulin. Some cats nadir early, some nadir later. +6 is only an average. ECID.