How do I find the centre line of my car?

[oily mitts]

I am trying to find the accurate centre line of my car in order to make some tracking bars.

The back of the car is pretty easy to sort as I can use the centre of the Hewland box as a center point for the rear end.

The front however is a bit more tricky as there are no clear measuring points to use to centre the tracking bars from.

I have dropped a plumb line down from the centre of the back of the gearbox, and also another one from a bolt on the underneath of the box which is in the centre. I have then aligned a laser beam using those two reference points under the car to a third plumb line off the front nosebox.

However the reference point at the front of the car looks a couple of mm to one side.

Am I missing something, or is there a better, more practical way to establish an accurate centre line?

Surely if the back is square, then if the front is out, even by 1mm, the car will end up crabbing and handle like a pig?

Any ideas folks?