In order to have default values for PET I have modified the default Prusa firmware.

Here is how I did:

You just have two files to modify. I have just superseeded the "PP" values as I don't use this kind of filament :

and then I made the same here

As you can see the "PET" settings are pointing of the PP_PREHEAT_HOTEND_TEMP. This does the trick!

And I did also modified the original "PET" to "PETG" to avoid confusion...