A little more information would likely be helpful. First of all, though it varies from one individual processor to another, the typical Tcontrol temperature for these processors is somewhere at or above 80c. A temperature of 70c is thus really not that high at all; your processor can be run continuously at its Tcontrol temperature for its warranted lifetime without suffering any degradation along the way. I would only be concerned if the temperature was this high while the system is sitting idle (which usually means either that the fan-heatsink unit is improperly mounted or does not have enough Thermal Interface material ("paste"). You haven't indicated whether this temperature was seen under load or at idle, however, so I cannot tell what is going on.
If your goal is simply to hold the processor to a lower temperature range, modifying the Minimum Duty Cycle parameter is not how you go about doing so. You should return this parameter to its recommended default (20%) and instead look at making changes to the processor's Control Temperature parameter. This parameter represents the top of the temperature control range. By default, it is set to the processor's Tcontrol temperature. On your board, like most of the 6- and 7-Series boards, the width of the control range is fixed at 15c. Below the bottom of the control range (i.e. below Control Temperature - 15), the fan is held at its minimum duty cycle. As temperatures increase across this range, the duty cycle is increased until, at the top of the control range (i.e. at the Control Temperature setting), the fan response reaches the maximum duty cycle.
A second parameter to look at is the Temperature Responsiveness parameter. This parameter determines the shape of the control "curve" across the temperature range. Rather than try to explain it in detail, it's easier for me to show this visually:
Try these parameters out and, if you have any additional questions, you can aim them my way...