Your correct that thickness plays a role in theremal transfer, but the paste and amount you use is not what determines that gap. Both laptop and desktop heatsinks are under quite a bit of pressure, more than enough to squish out extra paste. If you want to be extra sure you can spread a thin layer across the entire chip, but a dot or two usually works just fine. This video is on desktop coolers but mounting pressure should be similar.

If your laptop is over a couple years old I can almost guarantee there are dry spots on the chip currently. Laptops already run pretty hot and it's a double whammy for drying and pump out.