Lumens and foot candles are a measurement of the intensity of light to the human eye. They are not really relevant in the world of plants. PAR is the relevant measurement when it comes to plants and photosynthesis.
But anyway, to answer your original question. Reflected light is not nearly as good as direct light, but reflected light is obviously much better than no light. It is always a good idea to add reflective material to your walls. Mylar can be a pain in the ass to work with though, but very effective. Poly film is more durable, water resistant, reflects 90% of light (without being as harsh on our eyes), but is also more expensive.