Hey gherkin....I guess it all depends on the look you're going for, as to how much you chop it.

When we stretched the cab on the Stude, we took some side view pictures of the truck before we started.

We then got several copies made of the photos, and figured out the scale of the picture v.s. the actual size of the truck, and then cut the pictures so that we could try out our extension at different lengths, before actually cutting the truck.

Maybe this would work for chopping the top of your car too?

In my opinion, I think that a 2-3 inch drop would be good...but as I have learned on our truck, it's better to not cut off enough than too much...you can always cut some more if you think it needs it, and it's easier than tryin' to weld everything back together!

I'm not sure if all this made any sense, but I hope it helped a little...Good luck!