The show always starts around 9:00 pm, so unless you really linger over dinner, you will be done well before then. But, don't sweat it - the view of Illuminations from R&C is not significantly better than the view from many, many other great places around the lagoon (basically any place where your view is not blocked by a tree or a structure is as good as any other place). So, enjoy your dinner and then head out to enjoy WS. About 15 - 30 minutes before the show, start looking for a place to watch from; you will find plenty of good spots available then (some people start staking out spots an hour or more before Illuminations, but that really isn't necessary). Then, enjoy Illuminations - it is wonderful.