Below is one way of finding the average of a flat list, but let me know if I've misunderstood your question.
1. Create a list 'S' with only one item 'Pull'.
2. Create a module (e.g. 'Module1') with your flat list as the dimension, and line items called 'Value' and 'Mapping'.
3. For the line item called 'Value', leave the format as number and in the formula reference the line item of which you want to take an average.
4. For the line item called 'Mapping', change the format to the list called 'S' created in Step 1. Reference the single item in this list in the formula, e.g. " S.Pull ".
5. Create another module (e.g. 'Module2') with list 'S' as the dimension and a single line item called 'Average'. The formula of this line item should be " 'Module1'.Value[AVERAGE: 'Module1'.Mapping] ", which should give you the average of the flat list.