As was said work in degrees and do the trigonometry.Then you can work over whatever distance you like.Rear toe is .16 deg per side for Elise.2 for Exige,use .2 according to Lotus.
I think if you do the math the factory basis toe on factory rim sizes when they convert degrees to a dimension in mm.
Unless you have spherical bearings in good shape on your control arms your wheels move around so much more than those small differences you noted.
