Here's another chart for your perrusal. Chaz is close to being accurate when he says the chopped boattail has the same Cd as the full blown deal; it doesn't, but past 50%, you're really just splitting hairs.
zero------ Cd 0.488
10 %----- Cd 0.375
20 %----- Cd 0.299
30 %----- Cd 0.240
40 %----- Cd 0.194
50 %----- Cd 0.162
60 %----- Cd 0.135
70 %----- Cd 0.125
80 %----- Cd 0.118
90 %----- Cd 0.118
100 %---- Cd 0.115
And the spoiler might actually be HELPING the Cd, something about making up for the steep rear angle.
Here's an article from car and driver from the 70s where they discovered adding a spoiler reduced the turbulance from the steeply descending rear glass.
I know the comment is coming, so I'll address it now. Won't a boattail ruin the lokk of the car? Well, beauty is in the eye of the beholder for certain. The Aerocivic is ugly as sin, but being able to more than double his peak fuel economy is quite impressive.
Here's a thread where aerodynamics made a car look really good.
But if you don't want to go to extremes, you could try putting in a belly pan. Done right it can help quite a bit, and no one but your mechanic will know about it.
And as far as the "thanks" system goes, it's a fairly recent addition. Anyone who's been on a while will have plenty of posts, but not so many thanks due to the thanking option being less than a year old, while a newcomer will have a higher number of thanks compared to posts.
If you want to read it from the experts, read the threads from aerohead and basjoos. I'd say they're the aero experts and have covered a LOT of ground on several posts on this forum. In fact, on the botom of this page I see 5 similar threads mentioned in the similar threads section, and 4 are by aerohead and 1 is from basjoos.