1/2 a rice grain size is my recommendation. Just do the drop in the exact center and place the heatsink over it parallel to the chipset, this will automatically spead it out as needed.

I personally don't recommend spreading it out with a credit card or a razor blade because if the heatsink has an uneven surface you will windup with either too thin a coating or excess coating at the edges, thus not transferring heat at an optimal level.

Everyone has a different way of doing things, I have found this way to be most effective for me.