does anyone know this angle measurement off the top of their heads or do you know of anyway i can measure it?
i suppose the measure on both sides is the same...
work with me on the drawing.. it took 2 seconds, it's if you cut the bumper and stuff off the back of the car and looked at it directly from the back

 

take 2 peices of wood and form them to that angle then break out the protractor(compass,ruler dunno its been awhile)
im guessing this is for builting a box? if that is the case then you could just bypass that angle completely and do the 90* thing

 

I agree with BadMunky about using a 90-degree angle for that part of your (presumed) box. The amount of airspace you would lose is insignificant, and it leaves a lot less room for error in measuring the distances necessary to make it. Nothing sucks worse than fabricating pieces and finding out they don't fit. Otherwise, just use your trig functions on a calculator (But as mentioned before, make sure your measurements are accurate.)

 

I 3rd the motion. 90* is hard to screw up. If you make it some arbitrary angle like say 85*, you've got some major work to do when cutting the top and bottom sections, plus you've got to use sin equations to determine distances. Granted none of that stuff is going to cause the world to stop turning on its axis, but for the absolutely minimal gains, it just isn't worth the hassle.

My first couple boxes incorporated some funky angles for the section that goes foward toward the back seat. I learned very quickly that whenever you can avoid complex angles, you're better off.

 

you're right... it's really not worth the gains i guess.