What is the proper amount of heatsink paste
A very thin amount of heatsink paste (or grease) must be used. The only purpose of the paste is to fill surface imperfections and irregularities. Only a thin amount is used to maximize heat transfer.
If too much paste is applied, then the part will not be in close contact to the heatsink, and this will decrease heat transfer. Another mistake is to apply heatsink paste as a single blob in the center of the device, leaving the edges uncovered. This will cause the part to push out the paste from the center location as the part is being tightened down, possibly warping and bending the part.