Is there any way of setting a default deck for the Add Note popup?
Not exactly, but there is an option in the preferences to change deck depending on note type.
The first drop-down box controls how note types and decks interact. The default of “When adding, default to current deck” means that Anki saves the last-used note type for each deck and selects it again then next time you choose the deck (and, in addition, will start with the current deck selected when choosing Add from anywhere). The other option, “Change deck depending on note type,” saves the last-used deck for each note type (and opens the add window to the last-used note type when you choose Add). This may be more convenient if you always use a single note type for each deck.
What abdo said. But additionally, this can be quite useful if you want to add cards while studying:
If you open the add dialog during a review session, it will copy over the deck and the tags of the card you are currently studying to your new card.
Thanks - the point here that I didn’t understand was that the current deck (ie, the deck you last selected in the Decks screen) is selected by default in the Add Notes popup. The help section under ‘Adding Cards and Notes’ doesn’t make this clear.