Center That Guide

If you drag a guide from the ruler onto the page and you want to make sure it is centered horizontally or vertically, try this: Press Command-Option-; (PC: Control-Alt-;) to unlock the guides. If necessary, make sure that only the one guide is selected by deselecting every guide and then clicking on the guide you want centered. Then in the Align palette (Window>Align), use the flyout menu to select Align to Artboard. Click on the Vertical Align Center icon (or Horizontal Align Center icon, depending on the direction of the guide).