[lastportofcall]

That's a good way to go. 20' if you're going to tie your rig right into the topshot is also a good idea.

9" tag end could be a little short. Depending on the size of your dropper and how tight you fish you could wind up not having your crab on the bottom.

I'd suggest tying a bit bigger dropper (about 5") for your sinker and running the tag end 12 - 15" to your hook. Nice rig, and in my experience I've caught bigger fish with longer leaders (but not too long).