Dun know why you would need to put on thermal paste if your HSF already has it.... But that's just me. My Zalman didn't come with any so I don't know much about that.
But if you are going to put thermal paste on it, I would suggest using the "drop" method, where you just squeeze a tiny bit of paste onto the CPU. The pressure should take care of the paste distribution. I did it for mine and it cools perfectly.
It seems like you are too worried about the amount of thermal paste. In actuality, you only need a small amount because if there is too much, the sheer amount of paste will generate even more heat and there will be less heat transfer. Less is good.