Hi Christina, Praphul,
I think the histogram should be treated as a tool to answer specific questions only, I do not find it at all helpful to try to make a better image. Also be wary of the luminance/monochrome versions over individual, or overlaid colour versions.
Questions it can answer (particularly if separate RGB);
a) where is the black point?
b) are any/all colours blowing?
c) have I got room (a bit of flat space either side) to apply Local Contrast Enhancement
(LCE) without losing image detail in shadows (left) or highlights (right)
... and that's about it.
I view the entire image full screen, or at 100%, to answer other specific questions that arise in PP and I formulate these questions in my mind by looking at the entire image and see what strikes me about it;
i) good composition, does it need a crop?
ii) any distractions need removing?
iii) does it lack "pop"?
iv) what pixel size is it after the crop? this determines what size I should downsize to and post it at and it still look good (after sharpening)
v) is it sharp enough, or over sharpened?
My belief is that if you can't 'formulate the questions' and feel the need to resort to 'measurements' (even graphical ones) then your seeing too much wood and not enough trees/forest and you should stop editing (save it as psd or tif, not jpg) and return to it later. It happens to me too.
Practising critiquing other's pictures (as well as your own) and saying why you think something works, or doesn't, can be helpful, even if you don't actually post what you're thinking here. If you do, make it clear your purpose, we wouldn't want to offend anyone. By all means use a histogram (available in FireFox as an Add-on), but only to answer a), b) or c), not to try to emulate, for the reasons Urban, Dan, Steve and Mike have demonstrated and/or explained above.