Place a guide numerically using View / New Guide and Lock using View / Lock Guides or Command + Option (Mac) / Control + Alt (Win) + “;” . And finally, when dragging out a guide from the rulers, Option (Mac) / Alt (Win) toggles the orientation of the guide (vertical to horizontal).
The point of origin (or the starting or zero point of the rulers) can be changed by simply clicking and dragging the box in the upper left corner of the rulers (where they meet) to reposition. Double clicking at the intersection of the rulers resets the point of origin to the upper left corner of the open document.
In order to quickly find the center of an image, set the rulers to percentage. Setting the rulers to percentage can also be helpful when recording action that will be run on files of different sizes where you might want an effect to be applied to a certain percentage of an area.
To change Ruler units, double click in the ruler area to display the Units & Rulers preferences dialog box, or Control (Mac) /right mouse (Win) the ruler area to access units from the context sensitive menu.
To quickly display rulers at the top and left of an image, click the View Extras icon in the new Application Bar in Photoshop CS4 and select Show Rulers from the drop down menu, or use the keyboard shortcut, Command (Mac) / Control (Win) + R to toggle the rulers visibility. Command (Mac) / Control (Win) + “;” toggles the visibility of guides and Command (Mac) / Control (Win) + ‘ toggles visibility of grids.