I don't think there's a quicker / neater way to achieve the same end result as the fields for the 'AvailableLevel' colculation only come together after the Outer Join.
If ther they were already together you could use a preceeding load to get to the end result:
Quantity+Delivery-Dispatch as AvailableLevel;
From "Ready Merged QVD File";
But I'm assuming that's not an option.
Apologies I couldn't be more assistance.
All the best,
Matt - Visual Analytics Ltd
Qlikview Design Blog: http://QVDesign.wordpress.com