FBinNY 

I like to build mechanical knowledge the same way as a house by starting with a good foundation. So let's start by understanding what dish is about.

1- Wheels need to be symmetrical between the faces of the locknuts so the rim is centered in the frame or fork.
2- If a wheel is built with equal tension on both sides the rim will end up centered between the flanges.

In front wheels both conditions work together, because the hub is symmetrical. So centering the rim between the flanges automatically centers it between the locknut faces and by extension the fork.

In rear wheels both conditions can't be met at the same time because the hub isn't symmetrical. Because centering in the frame is paramount, we need to compensate for the hub's asymmetry by moving the rim off center between the flanges. Since the flanges are offset to the left, we move the rim closer to the right flange by having more tension on that side. This moving of the rim away from it's position centered between the flanges is what we call dishing.

So understanding the basics, we apply that to using the dishing tool. First and foremost, measurements are taken of the rim's distance from the locknut face since that's what determines the position in the frame. The wheel in the photo has the rim closer to the left, so it needs to be moved right by loosening left spokes and tightening right spokes. As a matter of good practice, you want to do this by degrees rather than trying to guess what it will take to move it far enough. Also, so as to work with looser rather than tighter spokes always loosen the spokes on the too close side (left for this wheel) before tightening the other side. If the rim was basically true before, preserve that by being careful to move each nipple the same distance, ie. 1/2 turn. Retrue roughly before checking the dish because you can't get meaningful readings if the wheel isn't true.