How to calculate a lithium-ion battery's discharge time at a specific load?
The formula you quoted is a good approximation.  It ignores a lot of things that will reduce the effective capacity, but it will give you an upper limit on the run time.

Given a steady discharge rate of 20uA, you are getting into an area where you need to consider the self discharge of the battery. 

Over the theoretical 90 days your example battery would run with a load of 20uA, it would lose maybe 10% of its charge due to self discharge.  That reduces your run time by maybe 9 or 10 days.