My Company Is Putting Together A Healthy Weight Challenge For

My company is putting together a Healthy Weight Challenge for our employees. The challenge will last 8 weeks. I only want to reinforce (with rewards) people who lose weight in a healthy manner. What is the maximum percentage of weight loss that would be healthy during an 8-week timeframe. I'm arguing for 12%, but I'm being told that is too low. So, someone whose starting weight is 185 pounds and loses 22 pounds during the 8 week timeframe would receive and incentive. But, if he/she loses more than that -- no incentive. What do you think is a healthy % of weight to lose during an 8 week timeframe?