Claire, I use the incident method. For the low value I point the dome towards the camera, for the high value I point it towards the light source. I determine the high values by getting closer or further away from the light source. This is how I did the refractory shot. This BTW was a 37 minute shot and I measure the highlight at the frame of the window...I wanted the "glow" from it to be infectios. If I wanted the window more defined I would have gotten closer to the window so that it would have indicated a higher value and the corresponding lower development.....