The Star Target amplifies the effect of gain, slur, doubling, paper movement, (35 to 1, if I recall correctly), so that small distortions in print can be easily seen by the operator. The indicator is visual; there is no way to quantify the result.
In trying to simplify the color bar I was designing for my approach to G7, I chose to eliminate all the mechanical diagnostic indicators that reported the condition of the press itself. I was interested in the printed result, mostly the gray balance. In the interest of making best use of the limited space, I replace the venerable Star Target with a visual indicator of tone and balance instead.