Suspension - the important ratios?

[Airhead]

Hi all

My build is cracking on and although I've not upadated the build thread yet I have been busy. I am at a point where I have planned the rear suspension mounting points by taking the measurements off an MX5 and replicating them on the rear frame. The wishbones will be the same dimensions as the donor ones so my track should be spot on; as a pleasant aside I am still within book dimensions so the rear tub will fit, fingers crossed. I have drawn the rear in my CAD package and then on Sketchup to prove that it works.

The front is a bit more complex...

Book has the dimensions between the mounting brackets as 670mm for the top, 494mm for the lower and 182mm between, the MX5 is 727, 667 and 178 respectively (there are some weird angles off the top tube as well with it being 4.5 and 2.5 degrees off true on two of it's three planes). I am not worrying about the exact geometry of the donor as I am using bespoke wishbones.

Question:

a) Should I be worrying about the variation on the top tube?
b) Moving the top mount down by 4mm and lengthening the top wishbone accordingly should be fine, shouldn't it?

Thanks for any constructive input.

Matt