I would really discourage her from brumating at that weight. Last year I had one brumate at 245 and was worried all winter. The other problem is that when females brumate they generally develop eggs when they awaken and at less than 200 that is very risky. My vet wanted to spay mine last year, I declined, but it worked out OK. Keep in mind, I have many dragons and experience though. I mated her to make sure the eggs were fertile to reduce the risk of egg binding.
But to answer your questions. I continue to have the lights go on and off on a timer. I slowly back the lights on time along with the normal daylight if you have only one dragon. It gets more complicated when you have many in the same room and some are juvenile on 14 hour days. I maintain the basking spot but try to lower the ambients on the cool side a little. Ideally I'd like a cool side to be about 70. As I stated before you'd have to supply a cool side hide and make sure it's used rather than a warm side hide. Cool temps. lower metabolism and allow them to brumate for long periods with very little weight loss. In your case I'd be real worried about 20 grams. I would also definately get a fecal done prior to her brumating.