You can, as you say, run a curve in the 12-16 range and then get the corresponding shaded strip response. But, when you run shaded strips later, how will you know how close you are to the limits? If you always get exactly the shaded arc height response you correlated at some value in between 12-16 then everything is ok. But in practice you will get varied readings of shaded strip arc heights. You need the upper and lower responses to verify that you are withing the tolerance band.
