Panic stop from 20mph? No such thing, unless you just weren't paying attention. But, to answer the question, like above, 0 feet. The distance you travel while waiting to react is FAR greater than the stopping distance from 20mph.
Using the rear brake. Yup, I use it all the time, just like they weren't linked. I always have more than one bike, and good habits are worth keeping. The thing I work on remembering is to ease up on the rear brake as I apply more pressure to the front during hard stops. The rear on any sport oriented bike becomes very light during hard stopping, and will easily lock. With ABS you know you got it wrong when the peddle kicks back at you a bit. On a non-abs bike that would be a locked rear.
I love having ABS for practicing high speed stops. Its a much safer way to practice panic stops. It sucks finding the 100mph locking point on a non-abs bike. But, knowing how much brake pressure you can apply during a panic stop is a great skill to acquire. I learn this max braking point on every bike I own, ABS just makes it a ton easier(and safer too).