Aircraft should not start outside EOBT +/-15, BUT that still leaves a very wide range of take-off times. One day you could start within tolerance, taxi and be airborne in 5 mins ie EOBT-10, another day it could at the end of tolerance with a long queue at the runway and realistically be airborne at EOBT+45. ATFM thus has a possible airborne window of 55 minutes from which they need to model en-route airspace capacity. This is why the push for more predictability, networked airports and more accurate planned take-off times. Slow progress, but progress is there.