Since the "Other" group has been created using table calculation, you will have to do additional calculation for it. The formula should look like below.
% to Total Cards Issued:
IF [Rank Cards Issued] <= [Top N]
THEN SUM([LM Cards Issued #])/TOTAL(SUM([LM Cards Issued #]))
ELSE WINDOW_SUM([Design/Other])/TOTAL(SUM([LM Cards Issued #])) END
Hope this helps.
Thanks Nachiket! I realized this shortly after I posted, but your reply helps me to validate.