To quickly find out the correct horizontal/verical values (for example relative to a barline) to use for a category, do like this:
- Create a sample expression with cleared positioning offset values, but that use the same positioning types as the category will use.
- Drag the expression to the point where it looks correct
- Bring up the context menu for the expression and select Edit Expression Assignment...
- Read the values at Offset from Alignment Point and enter them in the category positioning fields.