For a parabola to work at maximum theoretical gain (based on aperture size and feed efficiency), you have to be within about 1/10th of a wavelength accuracy. That's about +/- 1.25cm. You can build it much sloppier, but the achievable gain and pattern will suffer.
The feed is also critical as you don't want too wide a pattern resulting in "overspray" which reduces xmit gain, or too small a pattern that illuminates only part of the dish, resulting in low receive gain. I short, you need to match the feed type with the f/D ratio of the dish.
The best source of dish and feed info is at:If you're gonna grind your own antennas, I strongly suggest you get a modeling program. I suggest 4NEC2 ver 5.5.0 from: Drivel: If you just wanna improve the signal in very roughly one direction, just chop a 5gal poly-something food bucket in half, coat half the inside with aluminium foil, dump your access point on the bottom, and you have an instant hemispherical reflector. Not even close to ideal, but still useful.