I don' t want to take anything away from Bob H., but scoring 11, 10, 8, 5,0 has been mentioned several times in this thread.
That has been the point of several of us: X are now 11. Not bonus points, not X' s plus one, etc.
Though it is correct to score your card by adding up, as you go through the shoot a shooter can keep track of his score " in his head" by using the subtracting method. This can be helpful if you are not one of the score keepers and are that concerned about your score. Counting down gives you a better idea of the pace you are on than adding up as you go.
As I recall, the IBO score cards are punched so there really is no math until the end anyway.