If you already set up the gradebook category, you could consider doing the following:
Set the aggregation method to Natural for the category --> This will result in displaying the total max points score that can be achieved for the category and the student's grade as derived from that (Sum points achieved / Max points possible), which comes back as a number, but can be displayed as a percentage (source: https://docs.moodle.org/403/en/Grade_aggregation#Natural).
Set the aggregation method to Natural for the category --> This will result in displaying the total max points score that can be achieved for the category and the student's grade as derived from that (Sum points achieved / Max points possible), which comes back as a number, but can be displayed as a percentage (source: https://docs.moodle.org/403/en/Grade_aggregation#Natural).
If you know what the max points value for the category is, you'd be able to use max points / 2 as a value for the passing grade on that category. In the example above, that would be 15 points out of a possible 30.
For the student, their score will be displayed as follows:
or:
This should do the trick, I'd guess.