You are adding a strongly approve number to a somewhat approve number to get a total approval but you are only using the strongly disapprove number to show the 5% difference. Where is the somewhat disapprove to go along with the strongly disapprove?
That's what the "Abyss Index" (invented by a FReeper; if I knew who, I'd give credit) is. It's a way of pointing out how remarkable it is when Strongly Disapproves outnumber All Approves, both the Stronglies and the Somewhats.
The total Approvals are 42% (24% Strongly, and 18% Somewhat); total Disapprovals are 57% (47% Strongly, 10% Somewhat). In creating the Presidential Approval Index (currently -23%), Rasmussen tosses out the Somewhats, on the theory that they are less likely to vote, being somewhat ambivalent. Throw the Somewhats back into the mix, and the deficit for Obama is a still-dismal 15%.