I may do things a little differently than most (i.e., on the cheap), but it works really well, so I'll throw in my two cents' worth.
I do my graphics in one program (MS Paint), and print them in another (word processing program Open Office). When I've got the graphics the way I like them, I save them as a TIF image, then paste that into a blank document so I can easily size them.
Through trial and error, I've found that on my individual computer set-up, when the "view" setting in Open Office is 104%, the image on my computer screen is exactly the size that will print out. I take measurements from the relevant kit part and compare them to the screen image to make sure the fit will be right, adjust the size if necessary, and print away.
The advantage (for me, at least) is that I don't have to calculate anything, just "drag and resize" the markings the same way you would do with any picture. I often do multiple projects on a single sheet (keeps the decal paper waste to a minimum), and this lets me easily alter or resize an individual marking without affecting anything else.
Hope it helps.