This tutorial by Dr. Graeme Faulkner(retired Tasmanian Engineer/Teacher) walks students step-by-step through using View mode to calculate a light sensor threshold. Note, at the end of the video, the threshold is used in a switch block, rather than a wait-for-light block, but all the steps are the same for both.
http://www.drgraeme.net/DrGraeme-free-NXT-G-tutorials/LightSensorCalibra... [1]