Originally Posted by Talonmotorsport
Don't forget that the face of the rear upright is determined by the bottom rail of the suspension cage, the bottom wishbone and the rear upright it's self. If all these things are even 0.5-1mm out any where the worse case you could end up being 3mm out very easily. With the best will in the world the rear suspension is made from fabricated parts welded together not CNC made and assembled production components.
It's called accumalative error, if you start with the basics, each item will have a tolerence on dimensions, then if it is a welded component, each weld will have a tolerence too, once you start to add fabricated items together then all these tolerances can quickly become a problem if not kept in check.
I appreciate that this is a home built car, so keep the tolerances in the real world, it would be impossible to work to 0.1mm for example.
The way round it would be to have a jig for the suspension pick up points front and rear firmly screwed to the table and make sure that these are in alignment, then build the chassis around these. After all, the purpose of the chassis is to join the important parts (suspension etc) together.