Ok, lets back way up and get some very elementry UT fundamentals under your belt before we go any further.
Scale or Screen range. This is very important, if you are trying to determine where something is that you have found....if you don't know where your seeing, how will you be able to determine whether the indication is in the weld or simply the edge of a backing bar?
If you have your machine set for a 10" range, and you have an signal on the screen at 50% FSW (Full Screen Width), where is your signal?
(I won't confuse you with determining where in the part your signal is or which leg your in, for now, we'll talk about that later once we get some of this understood)
***edit: Sound path = %FSW times range
or in other words: range=10" x 50%FSW = 5" Sound Path***
answer = 5"
If you have your range set at 5" that same indication would show up on your screen at 100% FSW.
If you have your range set at 3" that same signal will be off the screen to the right so far that you won't even see it and would completely miss a flaw that could make the weld fail and hurt somebody.
Are you starting to see the importance of the setting the range yet?
The range has nothing to do with the sensitivity at all in this case.