There was a possible 12000 feet for jamming each gun: a 2000-foot course, with 6 test runs per gun.
The punchthoughs for that gun were all added together, and then subtracted from 12000 to yield total feet of jamming.
Then the percentage was calculated for that gun. If a jammer was able to jam 6000 out of 12000 total feet against a specific gun, it would be calculated as 50% effective against that gun. If a jammer was able to jam 9000 out of 12000 total feet, it would be 75% effective, etc.
The percent effectiveness was calculated for each individual laser gun, and then all of these percentages were averaged together to yield an overall average effectiveness percentage.
Is this a perfect way to represent overall jamming capability? NO.
On a shorter or longer test course, these numbers would be much different. However, it does give a good "one number" idea of how the jammers might generally compare against each other.