As Echo said, or more specific, the game picks up most stats - among them skilldata.txt - from a directory <divinity directory>\Data\Public\<modname>\Stats\Generated\Data.
This is where you should put the edited .txt file.
The game does not care about the Excel files, they exist to create the .txt files via macros.
The game does not care about the ...\Data\Editor directory and its subdirectories, only the mod editor does.
If you use Steam Workshop type mods which contain their own skilldata.txt, you can make as many changes to existing spells as you want, the game will not pick them up if it has already read the spell's data from the mod. You have to deactivate such a mod or unpack and edit it.
In an old style mod like 'Scales', you can put the changed file into the appropriate directory or edit the mod's file. Pretty sure Scales has it's own version of a skilldata.txt, so you would probably have to edit that.
