Whatever kind of mirror works best is the way to go.
This reminds me of some recent shaving improvisations during an electrical power failure. My bathroom does not get enough natural light for good visual shaving, so I tried propping up a small hand held mirror near a window. I got an acceptable shave, but when the power returned the bathroom mirror revealed several spots I had missed. I suppose I would get better at it if I did it regularly, and might even learn to shave well without any mirror at all.
I wondered what it was like in the old days without electrical light to shave daily with a straight razor. Many people must have shaved at times of day or night when natural light was not sufficient, or before sunrise, with candles or kerosene lamps. I tried to simulate this at night in the bathroom with a candle and my shaving was no better than semi-successful. In the old days there must have been many times when folks shaved with cold water, and some people preferred to shave during bedtime darkness to reduce the morning rush.
