The picture is subtle, but you want to fill the depressed area full of reflected light, but also be able to see distinct orange peel features of the paint surface. Here is one example of a sub-standard read. This is not the distance, but the degree to which the bulb and background are facing the dent. Sometimes a slight adjustment to the angle will make a huge difference in the read. Never make big changes here.
Subtle ones work best.
When working outside use a shade board to keep any glare from impeding the reflection. Remember-PDR is a process that is dependant on reflections. You are blind without them and the process will not yield good results unless you can adequately find your tool tip. Check back soon for more technical assistance. We welcome any calls or inquiries you may have as well.