Use the whole bottle. How much water are we talking about? And let it soak for as long as you can. Again, I let it go a week before dumping and multiple rinses and drying out.
Wow! Only three gallons of water! Might only want to add a cup or so of bleach....although the whole half gallon won't hurt anything, short of having to rinse it out in the end.
Nope. Its fine. After the stewing, rinse all the rock multiple times. How many? Until its driving you mad. Kinda like rinsing out new sand (I hate doing that). Then, let it dry out as completely as possible. This gets rid of the chloramines. Then soak it again in water and "Prime" to neutralize any remaining chloramines. Then your good to go. And your going to have some awsome Pest Free rock to start your new tank!
Just be sure to set up a QT tank (if you haven't already). Keep it up and running even if it's empty. This will keep you from the temptation of adding corals or fish to your DT. Usually you stop by a lfs and get an impulse coral or fish and then get too lazy or just dont have the time to set up the QT. Into the DT the new livestock goes and all the work you did to cure all the rock is meaningless.
So.... QT!! ;Jawdrop