my method
I'm sure you will get a fair number of responses. I don't think there is a defined distance that you should expect the spinner to settle, but it will settle.
My technical counselor had me make an "dummy" firewall, early on in my build, from two 3/4'' plywood panels applied together, to which I mounted the motor mount, Lord bushings and engine. We then let it sit, with the weight of the engine conforming the bushings, while I built on.
Later, I mounted the mount-plus-engine combo to the real firewall as a unit.
While I have had some minor settling, overall I am happy with how this helped the cowl to spinner cosmetics