A core plug of tight sandstone is saturated with water and measured to have a permeability to water of 2 mD. The core is evacuated, dried, and saturated with helium at the same state of stress. At an average pressure of 2.1 atm, the sample measures a permeability to helium of 7 mD. What is the slope of Klinkenberg's equation to account for gas slippage in mD/atm? Please include one decimal place in your answer.