I use multiplication for the box-blur to make the blur independent of the viewing distance.
The problem you get with your method is that when you sample the shadowmap near the (0,0) coordinates you will get less smoothiness then at the point where you sample the shadowmap near the (1,1) coordinates.
if you want to make the blur independent of the viewing distance you should do something like this:
float4(IN.Depth.x + 0.001/viewDistance,IN.Depth.y + 0.001/viewDistance,IN.Depth.z,IN.Depth.w)
