Precisely Position Guides in Adobe Photoshop
When it comes to work with guides Adobe Photoshop provides wonderful command New Guide located in View menu. This command opens small pop-up window with the same name where you can set exact position of your guide. It works perfect regardless of level of magnification or document size. Options that you have are Orientation and Position. Orientation can be set to Horizontal where 0 point is top of your document and Vertical where 0 point is left side of document. In Position text field you can type number followed with any allowed measurement unit in Photoshop: pixels, inches, centimeters, millimeters, points, picas or percentages.
Precisely move guides
Can I change position precisely after setting guides you may ask. Yes you can. Select Move Tool and navigate over guide. When it turns into double-sided arrows press Shift key and drag. Shift key will give command to Photoshop to snap to first available Ruler increment. First available increment can be any measurement unit set for ruler what you can easily change on right click on Ruler (Ctrl/Cmd + R). Depend on measurement unit set for Ruler you will probably need to change magnification level to higher (300% for example) because in low magnification levels Photoshop will snap to 12, 10, 5 or 2 points, pixels, millimeters from current.
Guides and Snap
Guides in combination with Snap from View menu really rocks and allows you to quickly and precisely position existing or draw new objects. Beside allowing Snap option from View menu you can check and what is set in Snap To > option.
Excellent and free plugin to create guides
Lastly to not forget excellent and free Photoshop plugin to create guides: guideguide which I am using all the time.
Photoshop / Grid and guides
Product used in this tutorial: Adobe Photoshop CS6