This is quite easy to do.

The Built in **Screencapture** has a command line option to capture a rect area. This is rectangle with a screen coordinate point and width and hight,
 x,y,w,h

We can use this in an Automator.app Service workflow and then give the service that it creates a hotkey shortcut in the keyboard services system preferences.


First we create the Service workflow in Automator.

Open Automator and a new **Service** Workflow document.

Set : 

**service receives:** none 
**In :** Any Application 


Add a **Run Applescript** action.

Type or paste this code into the **Run Applescript** action, replacing the default code.

      set theDate to do shell script "date +%Y_%m_%d_%H%M%S"
    set theTempPath to POSIX path of (path to desktop)
    set thePath to theTempPath & "screenCapture_" & theDate & ".png"
    
    
    do shell script "screencapture -R320,210,848,440 -tpng" & space & quoted form of thePath



 [![enter image description here][1]][1]



The code above will create a timestamp string to add onto the end of the file name.

This serves two purposes, 1, you can visually see when they where taken, 2 and more important, the files will no be overwritten by the next capture as the name will be different.


**-R320,210,848,440**  is the Rect option: x,y,w,h  

**-tpng**   is the file type format option: png


**& space**  we need a space between the options and the next part of the command.

**&quoted form of"  escapes spaces and illegal characters that the command line will error at if we do not guard against them.

**& thePath**  adds  the file output path which includes the file name

The ampersands **&**  are applescript syntax to say this **and** this.   


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Save the Service. You will be prompted to give it a name.

Now open the system Preferences and go to the keyboard system pref. Then the Shortcuts Tab and finally the services selection panel.

 [![enter image description here][2]][2] 

Scroll right down to the bottom to the **General** group

Select your service and give it a shortcut.

In most cases this will be picked up by apps straight away but some may need to be closed and reopened.


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An aside:

The files are set to land on the desktop.

 You can change this in a couple of ways. 

But to save me adjusting this for everyone, the easiest thing to do is add a **move finder items action**


First add this bit of code to the end of the code above:

    return (POSIX file thePath) as alias


This will pass on the file path to the next Automator action.


Then add a  **move finder items action** and choose your destination from it's dropdown menu.

[![enter image description here][3]][3]


  [1]: https://i.sstatic.net/wpGUd.jpg
  [2]: https://i.sstatic.net/kupB3.jpg
  [3]: https://i.sstatic.net/rsAk3.jpg