I've been comparing linedrawings of production cars with Cd's equal or lower than 0.29 and it seems on every single one of them the rear angle is between 15°<>16° for most of the lenght. it's also interesting to note that on some cars this angle goes even further down near the end of the windshield but at the end of the windshield theres either a spoiler lip or a trunk and if you continue this 16 degrees like it almost always meets with the edge of this straight piece.
this could be an indication on how long horizontal trunklid extentions should be.
here's a couple of them
they are not to scale and i've only looked at drag coeficients and not frontal area, obviouslt a good or bad cd is not only due to the rear angle
for example it was found that the citroen C4 coupé has a near identical rear slope than the honda insight although obviously the C4 is an otherwise more conventional car wich is less aerodynamic refined elsewhere
also it's always a gamble if values found on line are true