My husband was doing some work in Photoshop the other day and wanted to accurately measure a point form the centre of the screen. The problem he had was that the ruler is automatically set to measure from the top left hand side of the document where you see the two rulers cross. If you want to measure from a different point, it works exactly the same as Quark. Just click on the point where the two rulers meet and then drag out until you reach the place you want your measurements to start. Then let go and the ruler origin will appear at that point as shown below.
