Let's see:
ADR at 6pm, lets say 10 min wait, puts you at 6:10pm. Give or take 1 1/2 hrs to eat (are you on the dining plan?) puts you at about 7:40pm.
If you're going on the monorail, then you have to take a boat back to MK, monorail to TTC, monorail to Epcot. That's going to put you at Epcot around 8:40pm. By the time you walk to the bridge separating FW & WS that's right around 9pm.
If you go by bus from WL, you'll get there around 8:15pm - 8:30pm, again putting you around the WS entrance around 9pm.
If you go by car to WL then take the car to Epcot, you can get there by 8pm. Plenty of time to park and get comfy.
The beauty of Illuminations is that there really isn't too many bad spots. You can position yourself anywhere around WS and still have a great view. I've stood just in front of the monorail track on the walkway between FW and WS and still had a great view.