I would let her eat as much LC as she wants assuming she is not overweight and on a diet for that. Reasons: 1) eating can stimulate the pancreas to produce its own insulin, 2) when they are unregulated they need more food to sustain the same bodyweight (something about metabolism being off?), and 3) if there is some chance she is getting too much insulin, she will naturally try to balance that by eating more. So IMO food provides a safety mechanism, and also can help their pancreas along (several small meals or free feeding are the ideal).
I can see the reasoning in keeping the amount of food in check if you are feeding higher carb food, as it will lead to higher #s and then needing more insulin to balance that, making regulation harder. I don't think that applies much though to LC food, and besides I think the pluses of extra food outweigh that potential minus in the case of LC.
If you aren't comfortable just disregarding what the vet said, you could always give it a 2-day trial or something. If you feed her more and things seem more out of control, you can always go back to the vet's scheme.
Try not to get to frantic over the #s, I know it is hard. It can take some time - typically weeks and sometimes months to get regulated. It is rare that it happens in a few days or within a couple weeks. If you can keep yourself breathing and taking a slightly longer-term view of the #s (i.e. don't let each # freak you out), you may find it easier to cope with. I know, easier said than done.

I'm sure it is extra-scary with the ketone threat. :YMHUG: