How can I measure PX distance between two points on the fly?

I am a programmer, not a designer. So I often need to know how many pixels a certain element is far from top or another element. But I am not sure how to measure this on the fly in the Photoshop CS6.

I changed the ruler to Pixel values. I then zoomed in and made two guides between two points I want to measure. It looks something like this

Is this a proper way to measure the distance? If yes, how do I measure it now? I am sure that Photoshop has a better option than that I have to count pixels from the rules by subtractions.

If this is the wrong way, what is the quickest way to measure the distance between two points on the screen?