[Krait]

The chassis centreline should be established first by finding the centre between the inner pivots of the front lower control arms and then the inner pivots of the rear suspension lower control arms, am not too familiar with Jags. Permanently mark these positions for later reference.

Using these two points you can run a string or a laser between them to determine the vehicle centreline. Measure off this line to accurately place your sidestrings.

As Bob says an aluminum tube front and rear sitting so that the side strings are at wheel centre height works well. You can notch the tubes so the string sits in the same place each time, for repeatability.

Cheers