This sounds more complicated than it is. Just look what happens through the viewfinder to the scale as you change Exp Comp.
David - My choice of Aperture priority is usually driven by creative decisions rather than light. I'm mentally thinking about DOF and whether I want a lot or a little. And I'll use ISO to make sure that the shutter speed is reasonable.
I use Shutter priority in a similar manner. In practice, I stay in Aperture mode even when I think in terms of shutter speed. I just adjust the aperture and ISO to reach my desired shutter speed.