notice the rear spoiler.. draw a line from the top of the rear window to the top of the spoiler.. that angle is far slower than the angle on the stock CRX.. the CRX had a spoiler to cheat a bit also, however actually having body panels in that open void would be better as not vortexes of air can form..
That imaginary line on the camaro.. is just about what angle is pictured in the mockup for my CRX
As fo the insight you have to look at the entire length of the car.. not just one particular angle.. the mx3 is the same shape as my concept.. and insight have concaved window. if you look at them..
MX3
My concept is lessening the flat portion at the rear by extending he tail longer.. making for less area for the air to create draft.