For this tutorial I used K3CCDTools and my LX200 with Meade 0.33 Focal Reducer and Scope Teknix Filter Wheel and my ATK16IC camera and imaged M42.

First I measured in Megastar - but you can do the same in Cartes du Ciel etc. - the distance between 2 significant stars.
I have put a * to indicate which stars I used for the measurrment.

Megastar shows a distance of 4.1 arc minutes (at the top of the screen print).

Next I went to K3CCDTools Options|Telescope and CCD camera.
Make sure you use the correct values for the pixel size of your camera and the frame sizes !!

Finally I measured in K3CCDTools the distance between the 2 stars of M42 (I used the stacked and stretched result which I had saved as a TIFF file):

K3CCDTools told me on the status line: 4 arc minutes and 10.64 arc seconds, which matches what I found with Megastar.
Thus the focal length of 1270mm of my optics+camera combination is correct, and the FOV of this combination is 12.82 x 9.61 arc minutes.
To arrive at the correct focal length - which I had already done before - is an iterative process: you have to vary the focal length and measure again in K3CCDTools till the distance between the 2 stars matches the distance in Megastar (or Cartes du Ciel etc ...)

I have made a whole table of my different setups, which I very frequently use to select the most appropriate optics+camera combination for a certain object.
In Megastar and Cartes du Ciel you can define your FOV, so that you can compare the preview on your screen with what you were after.

Here is an example: I wanted to image the Galaxy Triplet in Leo and quite some time ago I had already found out that I needed my ATK16IC camera and WO ZS 80ED + 0.33 Focal Reducer to get the triplet in my FOV; I store this kind of information in my ToDo list:

So in Megastar I used this setting:

The following image shows the match between Megastar and a real image (just mouse in/out):

My FOV table is here and I absolutely recommand that you make your own table too !