Save highlighted text as int variable
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
quits

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:
#!/bin/bash

x=$(xclip -o -selection clipboard)
xclip -i -selection clipboard &lt;&lt;&lt; $(bc &lt;&lt;&lt; &quot;$x + 0.25&quot;)

These steps are, however, outside the scope of this site.