you have to remember that there is very little clearance between the heatsink and the CPU head spreader (literally like putting 2 dvds together). the only reason you put the thermal compound on is to fill any gaps and have a compete connection. 1 line should squeeze and spread right out super thin. if you put to much, it will squeeze out the sides and could contaminate critical circuits or other components. This is a case of More isn't Better. Its a case of, Just the Right Amount, not to little and not to much.
I normally put about the size of half a grain of rice in the centre and then wearing a disposable rubber glove, use a finger to evenly smear across surface of chip, works better than using the force of the heatsink being tied down.
the heat sink should actually directly make contact with the heat spreader and the thermal paste is actually only for microscopic imperfections. In general the less thermal paste the better. About the size of a grain of rice is plenty just as das_stig said.