When designing in Photoshop I frequently place a guide a certain distance away from an object so that I can keep consistent spacing (e.g. all headings could have 40px of padding below them). In Photoshop, I can use the Ruler tool to measure 40px from an existing object/guide, than drag a new guide out, let it snap to the end of the ruler, and release it.

In Illustrator, though, the Measure tool disappears as soon as I start to drag out a guide.

Is there a way to place a guide (ideally) or an object itself (if guides aren't possible) a certain distance away from an existing object or guide?