Killing 8 Divisions out of 18 is more than 10 percent!
If they were killed. They weren't. And decimated does NOT mean 10%. It means EXACTLY 10 out of EXACTLY 100. Not 9 out of 90, or 1 out of 10. Decimation was a unit punishment. If a century was to be decimated, and they were not up to strength, they would be brought up to strength and decimated. Pity the poor soldiers that got THAT transfer!
They would count off by tens. The centurion would call a number to step forward. The man to his right (or left) would be ordered to kill the man who stepped forward. THAT is decimation, and it has NEVER been done in modern times.