Comment by HeyLaughingBoy

You should never plan to use your safety margin!

That "extra" 400m of grass? That's for all the other things that can still go wrong even when you follow procedure. e.g., you're below V1 so you abort takeoff, close throttles and hit the brakes. You should be able to safely stop on the runway.

But now your brakes fail because maybe the reason you had to abort was a fire that also managed to burn through your brake lines, or it started to rain just as you were taking off, or...

Now that's where the 400m of safety margin comes in to save your ass (hopefully). It's "extra", you don't plan on using it.