The Pratt measure requires a "gold standard" - that is, something that gives you perfect or ideal edges for your images. You could use a standard edge detection algorithm for this purpose if what you want to do is compare another your edge detector with the standard one. The "ideal edges" are, as you say, the edges from whatever standard you've chosen.
The distance between an actual and an ideal edge is just the distance in the image, computed using Pythagoras' theorem on their coordinates. This assumes that you have some way of matching up corresponding edges, which may not be trivial.
There is a single summation because you sum across edges, not across pixels.
I'm not sure what pseudocode would add to this.
I recommend looking up Pratt's original paper or book. It's likely to be more understandable and give more details than the very brief summary in CVonline.
