You have to write an application for your PC that
- reads a number from the clipboard
- adds some constant
- puts the result back in the clipboard
Then you have to configure you desktop environment in order to have this
application launched from a keyboard shortcut.
Once you have done that, you can do the whole operation from a regular
keyboard: Ctrl-C + Launcher shortcut + Ctrl-V.
If you can do it from a regular keyboard, you can do it from the Arduino
emulating a regular keyboard.
Edit: The way you do the first two steps is very dependent on the OS
running on your PC. On Linux, for example, the mentioned “application”
can be a minimal shell script:
x=$(xclip -o -selection clipboard)
xclip -i -selection clipboard <<< $(bc <<< "$x + 0.25")
These steps are, however, outside the scope of this site.