[OmikronWarrior]

Another thing, there are ways to maximize odds when site searching. Depending on your initial site frequency set when the game was set up, each province is likely to have roughly 2 or fewer sites. Having 0 or 4 sites is statistically unlikely. Hence, if you've already searched a province for one magic path and did not find any sites, then its likelier for other path searches to reveal some sites. However, if a province already has two or three sites found in it, then the odds of any additional searches being successful go down. At 4 sites, the odds are zero.

Does anyone know the exact mechanic for assigning sites?

Edit: Astral Sites don't seem to have any preference for province type. However, I have had some luck generating additional astral income by using priests to site search. Most holy sites generate astral income. If nothing else, it gives your priests something to do when not needed to call back a Pretender or preach.