OK, I had misunderstood then
the calculation should be:
Based on multiple of column subtotal
subtotal of column 'MyColumn' times '0.5' 'plus' '0'
if you are still having a problem with this, create a new php page, and add a new cart display manager to it.
on the second page of the display manager wizard, set it to display all columns.
send me a link to this page and an item i can use to test.