An auto-entered calculation should work here and GetAsNumber should not be necessary.
I can guess at one possible scenario as to why this appears not to work when using a calculation field does display the desired value:
Try creating new record with the auto-enter calcualtion in place. I predict that the value will correctly appear in the newly created record. Existing records will not automatically auto-enter data when a field is modified to get an auto-enter calculation or the calculation's expression is modified.
See this thread for a way to update your existing records after adding/changing an auto-enter calculation:
The other issue that can be a factor is that changes to the data in the default table will not automatically update the values in existing records in your main table. You may need to use Replace Field Contents or the above method to update values when this is needed.