as a first step, under Flashcards > New Test > Display > Top Field 2 you could set Session Correct %, as here:
This will give you a live display of the percentage of questions that you answered correctly at the top right of the screen as you are being tested. You will know that when this value is higher than 85%, you will need to add new cards to the mix (i.e., select different categories), and if it's lower than 85%, you will still have some more studying to do with the cards you are currently being tested on.
This reading applies only to your current session. If you start afresh on a new day, for example, the reading will be calculated anew.
Thanks for the info. Just so I understood correctly, this does not affect the way cards are presented to the user? I'm more interested in modifying the way I get spoonfed cards so that the daily revision is efficient over the long term
You're welcome. I think if you set Pleco to Repetition-Spaced, you should already be seeing cards with a lower score more often. So maybe you can simply trust Pleco's default settings, just making sure your cards are at the right difficulty level approximately, and you don't select too many cards at once. (I would start with 200.)