The "proper amout" is going to be determined more by the flatness, or lack there of, or the IHS and HS. If they are both flat, then grain of rice is easily twice what you need. Most IHS's are concave so grane of rice or more is about right. I have put on just about every kind of thermal paste you can think of and I have put it on in about any way concievable. And from my experience, which I have lots, no matter how I put it on, dab in middle, line, spread it, squish it with HS, it never made more than a couple degrees C difference. And what I do now is just put some on a go, I dont even think about it.
As dood said too little, trust me its hard to put on too little, is bad, but also make sure your not putting on a toothpast size line..hehe.
just put a line in the center, and go with it. when you put the HS on it pushes the air out the sides. I personally dont like to SPREAD AS5 cause its really easy to make ridges that trap airbubbles.
People are far too worried about it. Unless your going for a world record OC, your worrying for not. If your running so far on the ragged edge that that 2C will make or break you then.....well your OC is going to fail and crash your system in a week anyways so who cares. hehe