Why not use Texture Coordinate > Object?
Hey, very nice tutorial... I am working on a heat distortion animation, but unfortunately it is more complicated but doesn't look very good (yet?). Yours is so much easier to do and still looks great... only question I got here: when you're centering the gradient texture on the plane, why add Combine XYZ and Math nodes, when you can simply use Texture Coordinate > Object instead of Generated? The gradient texture is automatically centered and you don't even need a Color Ramp node to get a smoother transition from black to white (if you're not going to tweak it to your desire). Since you're not going to move the center anywhere or do some other texture mapping, your node setup is overkill ;)