Paul,
Generally speaking, in determining middle grey you should take the middle values for RGB, i.e, R=G=B=128, or 256/2. In your image the jacket might be a good starting point, or the sack by the burro's rear side. I think the street payment might be a bit too light for middle grey. Hover the eye dropper over what you perceive middle grey to be and read the RGB values on the "info" tab.
Keep in mind, though, that by drastically changing the colour in RGB you're also affecting contrast. The way to avoid this is by changing the blending mode of any colour corrections layer to "colour." This will process the colour portions of the layer only and not affect contrast. If you want to affect contrast only (and not disturb colour,) change the blending mode layer to "luminosity."