In the Photoshop tutorial, Richard Harrington gives some tricks showing how to use calculations to make a selection. Selections are used when you need to make an alpha channel or composite layered images. The more accurate you can get your selection the faster you can get through your compositing work. This is part one of a three part tutorials series on making selections inside Adobe Photoshop.
This tutorial is also available as a podcast:
Click here to find it and many other Photoshop podcasts.