When I want to achieve what you seem to want, that is to scale (or extrude or grab) by a fixed distance, I do not use the scale tool, but rather, I make use of the values in the transform panel in the tool shelf (the N key) in the 3D viewport in edit mode, and plan to do a little math. In the case you present, what I do is to set the pivot point to the median value of the part I want to scale, and make note of the co-ordinates (x, y, and z, as needed) of the median point, with all of the vertices selected that I want to have moved. Then I move the vertices making up the edge or face on one side of the object to where I want them, and note the difference in the value along the axis. So, suppose I start by moving one side along the y axis. Having made note of the median value, I determine the difference between the y value in the new location, and the y value of the median point. Then I select the vertices on the opposite side of the first. In the box for the value for the y co-ordinate for the selected vertices, I subtract the calculated difference between the old and new locations. So, if the difference between the locations on the y axis was 2.5 in one direction, I select all of the vertices on the opposite side that I want to move, and subtract the 2.5 from the y co-ordinate of those vertices. Then to do the vertices in the perpendicular direction to the first, I select all of the vertices on the x axis (in this case) and add 2.5 to the value in one direction, and when I have selected the vertices for the opposite direction, I subtract 2.5 from their median value.
This is the most effective way to achieve what I understand you are wanting to achieve.