Hi Philip,
This is classic, this is what needs to be done. Just so you know, the trick is to create a vertical table instead of a Cross Table and format it to look like a Cross table.
Create the variables: A, B and Mean
A =Sum([Measure]) where ([LOB]="A")
B =Sum([Measure]) where ([LOB]="B")
Mean = ([A]+[B]) / 2
And then, create a vertical table with columns
.....(all the columns you need), [A], [B], [Mean]
Make sense? Let us know if this helps or if you have any questions.
FYI: I've done this a ton of times, you just need to create few more variables, but its ok.
Update: You may even be able to do it with a cross tab, by using the summary column to show the Mean numbers, we can help better if you can atleast add a snap with mock up data in excel.
Thanks,
Mahboob Mohammed