i would leave your room at 8pm. you need time to walk to the bus stop, stand in line, wait for the bus, ride the bus, walk to the entrance, get in, and walk to the restaurant which is a bit of a walk from the entrance. They will hold your reservation for 15 min past the time, so dont freak out if you are late.
It can be hard to sleep in at the hotels, especially if you have neighbors who are up and showering... only my husband can sleep through that!
One tip, go to the safari ride, which is very close to the restaurant and get Fast Passes, b/c once your done eating your FP time will be ready for you! We love to do that!!!
Eat some cheese blintzes for me!!