Placing a guide in the center of an Image or document in Photoshop may be quite tricky. But after this tutorial that wont be so hard at all. We are going to use Photoshop actions for doing this job. To do this open up Adobe Photoshop and make a document of any size/resolution. Show your rulers by pressing Ctrl(Win)/Command(Mac) + R on your keyboard.
Now we need to change(if already not of course!) the preference for Units & Rulers from any other to Percentage.
To do this double click on the ruler and the Preference menu will pop-up. Then change the Units to Percentage.
You can do this simply by Right Clicking on the Ruler and select Percentage.
Next we will record Action. Press F9 on your key board will open up the Actions palette. Choose Create New Action and a a window will pop-up. Name it Guide in Center(or as you want) and click on Record.
Now the magic starts… Click on View >> New Guide a window will pop-up again. Just select the Orientation and enter 50% and you will get a new guide at the center of your document.
Now stop recording the action and play it whenever you need a guide in the center of any document. You don’t need to set the Rules and Units to Percentage again for this.