You are doing it the same way most experienced builders do.
The idea with any thermal paste is to get the least "usable" amount down. Any extra is actually bad. You want a layer that is transparent, that's all.
The purpose of thermal compound is to fill the small cracks and wholes, to keep air out.
A mating surface of metal conducts heat away, much better than any paste can. This requires a flawless lap on bothe hs and heat spreader, which I do not recommend. A wee bit of AS can fill the flaws, without too much loss of thermal conductivity. You should not use more than is needed to fill those flaws.
A grain or rice is larger than the amount of AS that is used by most experienced builders.