What are the chances of an Unown spawn in a neutral biome?

So, I've been catching Pokemon for ages now, but I've only encountered one Unown. I still think I am very lucky to have even seen one, but what are the chances to have an Unown spawn in a neutral biome?

A neutral biome is a biome which a Pokemon may spawn according to the probability of spawning without regards to its location.


Solution 1:

From my experience there is once or twice a week a unkown in an area of 700km². Also the time and the location where the unkown spawns is random.

Reference: The data comes from a map which scans an area of 700 km²