There is some guidance on the national website:
http://www.scouting.org/awards/centennial/14-175-2.pdf
And the official form has some clarification points on the back of the sheet.
I certainly agree that this format is open for abuse, but the abuse must be sanctioned on three levels. The Pack level, the Unit Commissioner, and the District Executive must all sign on the dotted line. If you lowball the numbers to something ridiculous, like 25% retention, hopefully someone at one of those levels will call you on it. I seem to remember hearing something about "A Scout is Trustworthy