There's 6080 feet in a nautical mile, half that is 3040 feet.
Tha tangent of 30 degrees mulitplied by 3040 gives you 1755 feet (Tangent of an angle is opposite over adjacent in a right angle triangle), so its actually abit better than the 1500 I very roughly rounded too.
I'm certainly not advocating that steep of an approach, as any groundspeed will give you a very high rate of descent, I was just questioning my old buddy Xnr's 30 degree claim. It seems extreme to me.
Using the same math 500 feet at a half mile gives you a 9.34 degree slope, which is about what I would recommend.
If my math is off, and it works out to something more reasonable, I'll buy the first five rounds next time Xnr is in town.