Dude, I wrote that article about 13 years ago. I would actually have to re-read it to be able to answer you!!!
400m are normally run at about 80-85%... best idea is to time yourself on one all-out sprint on the distance then shoot for a time that is 115-120% of your all out time. For example if you do it in 75 seconds shoot for 85-90 seconds. Even if you are off by a few seconds don't worry.
For IBUR don't worry about where you are starting it intensity-wise just strive to improve over time.