When in USB or LSB mode, the filter skirt displays the low and high pass values as well as a center value. For example in USB with the 2.7 filter selected, the skirt shows 100 at the low end and 2800 at the high end. The meaning of these is clear. It also shows 1.5k at the center-top of the skirt. What does this value represent? How is it calculated? How do you use it? Thanks.