Don't get your hopes up.
It's pretty popular to assume that dicks will increase a proportioned size in comparison to the flaccid member. By your rule of thumb, you figure a 9" flaccid dick will expand roughly 50% to its 12-13" erection.
I think penile architecture and blood flow to the region have a tremendous impact on how much it grows (or doesn't grow) to its erectile state. A member on here reports having a 3" flaccid member that expands over three times its length to 11" when hard. And a personal friend of mine hangs about 6" (according to his ex-girlfriend) and only grows to 7" max when hard.
So, remember: flaccid lengths don't always accurately describe what you'll get when hard.