I'm cruising the forum trying learn more about cutlists and stumbled on your old question, which I just worked out
I'm pretty unsteady with this whole area just yet but
I think you have to use a BOM cutlist, not a weldment cutlist, then you can add a column equation.
The far right column is the TOTAL LENGTH cut list property. Notice it is different to my calculated Total Length, it is the Total Total if you like.
Im on SW15SP4
Better late than never, might help someone else