I've also done this run a million times via taxi, car service, subway, bus, horseback. Each requires you to plan for 45 minutes to 1 hr. As mentioned above, so long as you are in the security line at 8:30, you're fine.

I've left midtown 53rd&7th at 6pm and have made the 7pm, but that's really pushing it. If I get outside and traffic is horrendous, I hop on the N or W to Astoria Blvd and catch a cab, though cabs are sparse there forcing you to jump on the M60 bus. Best bet if traffic is bad, hop the N/W to Astoria (takes 15 to 20 min from midtown) and have a car service waiting there for you.