Eloquent Scope for average result?

I'm following along with Eloquent Performance Patterns, which is brilliant, and decided to tackle something of my own.

At a high level, A Student can have many Enrollments. When paginating the enrollments, I'd like to include a scope that contains the student's average percentage completion of all their enrollments (Thus on a front end table there could be a comparison between that course and the student's overall progress)

Additionally, I'd like to be able to pass a variable to limit this calculation to a given subset of courses.

class Enrollment extends Model
{
    protected $fillable = [
        'student_id',
        'course_id',
        'percentage_completed', //A float, where 1.0 would be 100%
    ];

    public function scopeWithAvgPercentageCompleted($query, $course_ids)
    {
        $query->addSelect(['avg_percentage_completed' => function (Builder $builder) use ($course_ids) {
            $builder
                ->from('enrollments')
                ->selectRaw('ROUND(avg(percentage_completed) * 100, 2) as avg_percentage_completed')
                ->whereIn('course_id', $course_ids);
        }]);
    }

}

However, when I run the query, I'd get very weird numbers for avg_percentage_completed e.g. Values greater than zero, even though the student has progressed through nothing.

//Show the enrollments for students with ID 1, 4 and 6 in the Course IDs for 123, 456, 789
Enrollment::whereIn('student_id', [1, 4, 6])->withAvgPercentageCompleted([123, 456, 789])->paginate()

I assume I'm looking at this the wrong way, but any pointers would be appreciated.