I was just thinking...
No matter what, the traction (thrust) on the two rear tires is never equal. The tire with the most traction will want to steer the car to the opposite side. To keep the car straight, the steering is adjusted, so to at least a minor degree, the car body is travelling a little "sideways".
If you look at the attached caveman wall painting (MSPaint), the red arrow is straight ahead, zero toe. This will still cause a little steering offset to the right when that is the "dominant" tire. Green would be the absolute worse case, and require huge steering offset to the right. The Blue arrow (exaggerated) is toe-in. At some point between the red and blue arrows, there is a point that will require no steering offset to go straight, hence keep the body perfectly aero.
I don't think it could actually be calculated exactly, but it seems there is a toe-in number that should actually increase your speed. The more "perfect" your weighting, tire dia and pressure is at full engine torque, the less toe it will take to make the body straight. CG location probably comes into it.