In your example you want to refer to that cell with a total of 584 for Category B. SSRS doesn't have the option for you to refer to a value within multiple groups like that. You can only provide one scope override. To get this functionality you can add a subquery to your dataset that aggregates those values in a new column.
So for example your dataset should end up looking like this:
CategoryName SubcategoryName Number CategorySubtotal Category B subcategory a 125 584 Category B subcategory b 458 584 ...
Now you can easily calculate the percent of total for each category in the report.