The Pixel Margin parameter allows you to determine the margin of the resulting UV map in pixels of the active texture. In order to use this option set the Pixel Margin parameter to a value greater than zero. In such situation the standard Margin option will be ignored and packer will try to determine the correct pixel margin. In order to make sure that the packed UV map will have exact pixel margin one should also set a proper value of the Adjustment Time parameter. During this time packer will be determining what margin should be applied to the given UV map before packing, so that the resulting margin after packing is equal to the requested number of pixels. Such adjustment is required because in a general case the UV islands are scaled after packing is done (so they fit the unit UV square), so is the margin applied to the islands before packing. After the adjustment phase is done the actual packing process will begin.
A rule of thumb is that the longer the adjustment time the more accurate the resulting pixel margin will be. In practice the adjustment time set to one second should be enough for a usual UV map. Set the adjustment time to greater values only if the resulting pixel margin is not accurate enough for the given UV map.
Note that the pixel margin adjustment phase is not needed if the UV map is packed without scaling (Fixed Scale option is on). In such situation the Adjustment Time option will be ignored.