You're pushing it but only a little. That's about 30 miles. You're starting out on the right side of town and there's pretty easy access to the Roosevelt Bridge once you are in a taxi and you'll have the advantage of HOV on 66 and the IAD access road. So worst case that puts you at the airport around 430 or 445.

BUT security lines at IAD, especially that time of day, are notoriously unpredictable even in the elite line. I've waited 30 minutes in ELITE security line that time of day. That's why I got Clear. Worth every penny for situations like this.

Then there is the 10 or so minutes waiting for and then on the People Mover.

So you'll really only have a problem in the worst case scenario, and if you're lucky you could be at the gate by 445.

If it were me (and I didn't have Clear) I'd try to slip out of the meeting a few minutes early just to be safe.