How do you determine coilover mounting length? For instance if the compressed length is 9in and the extended length is 13in, how far apart would the mounting locations be?
No, I say you allow 1" droop from full extended length. So, you'll have 12" mounting centres at fully loaded ride height, allowing you 3" compression and 1" rebound.
That is how the manufacturers tell you to do it.
The normal way to build the car would be to make dummy struts with holes in 12" apart, and build the car around that ride height.