As there has been around some quite colorfull ways how people approach the mathematichs of SB, in paint.net and other advanced programs, I decided to do this little tutorial to show how it should be done.
Offcourse this is the only rigth way
Step 1
Find the correct length of the object you are drawing. Wikipedia is used in this examble, but be aware, the usuall method would include douple and triple checking it from various sources. Expensive books and so on...
Step 2
now, we have 6760mm or 6,760m length for a truck in FD scale which is 1m = 22.093 pixels
Simply multibly 6,76x22,093 and you will get 149,34868, thats how many pixels its length would be in FD scale.
Step 3
Now, measure the same length in your source drawing, and in here we get 192 pixels. The size of the source and ammount of pixel haze makes this part the most important, specially the smaller your source picture is and the bigger the scale coefficient is.
now, divide your desired pixel length with this measure and you get: 0,777857708 which is the ratio your picture needs to be resized to get the lenght of the truck correctly scaled
Just multibly the width of the whole picture with that ratio and you will get 622,2861667, which is how many pixels the source picture needs to be to fit the truck in correct scale.
Now, the Paint.net and no one else is not going to get any more accurate in the scale than 1 pixel at this point (plus naturally the all past errors that multibly themselves in the previous phases). Use normal rounding rules, and thus in this case we set it in 622 pixels.
Step 4.
Now rezise the picture. In this case we came up with 150 pixels in the actuall scaling, so most likely the error is in the selection meassurument, or in the source drawing not beeing straigth. +/-1 pixel is around best we can get, so I wouldnt go changing that any more
Its now done, and now you can copy/paste the needed part to an different layer and crop it as much you want and/or do what ever your modus operanti with the layers/templates is. I myself would copy the simple truck into my paint worksheets and only after its completed, i would Place it on templates, but thats up to everyone's own choises now.