One method would be to use an expression to link the Bulge Center value to the Null position. In the Bulge effect controls, hold down the Command button on your keyboard and click on the stopwatch. You’ll see the number turn red, but you’ll also see that there’s now some text that appears in your timeline… something like ‘effect(“Bulge”)(3)’. With that text highlighted, click the pick whip button (the little spiral to the left of the text) and click-drag that to the position property of your null. It should bring up something like:
but just make sure that the “Null 1” is the correct name of the object that has the motion data on it. That should link the position data from the tracking to the bulge location.
It worked really well! Just one more question: is there any way to have the bulge’s horizontal radius/vertical radius change in relation to the tracking? When the camera zooms in on the eye that I’m “bulging” the size of the effect doesn’t change. I tried using your code on the horz/vert radius but it didn’t work.