Hello and welcome!
Firstly I'd like to clarify something. When you say "window shades" are you referring to the shutter type shades on the outside of the window, or do you mean the blinds that typically hang on the inside of the window?
A picture paints a thousand words, so here's a few thousand words ....
Bermuda/Bahama Shutter (image from shutterblinds.com)
Venetian Blinds (image from howstuffworks.com)
Vertical Blinds (image from howdididoit.com)
Roller Blinds (image from made-in-china.com)
As you can see,
window shades can be a little misleading! If you can let us know which type you are referring to, we can be a lot more helpful!
For what it's worth, most of these blinds normally come with their own mounting hardware, and they are usually different across different manufacturers, and even across the model range from a single manufacturer. You will probably have to remove the existing mounts and use the ones that come with your shades instead, so simply take your measurement as enough to land your mounting hardware approximately where the existing hardware is.
Hope that helps, and please let us know which specific type you are looking at so we can be more useful to you
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