The following is an updated version of a post I wrote about a year ago to explain fairmath to someone else:
The complicated thing about Fairmath is understanding that increases are based on how much you still have to gain, while decreases are based on how much you have to lose.
So if you have say a starting strength score of 40%, then there is only 40% you can lose, but 60% you can still gain. Therefore a 50% loss from a score of 40% will be smaller than a 50% gain, because half of 40 is less than half of 60.
40+(0.50*60) = 40+30 (50% of what you didn’t have is gained)= 70
40-(0.50*40) = 40-20 (50% of what you had is lost)= 20
The difference is even more dramatic if your score is very high or very low. Lets say you have a starting strength score of 90%, then you have have only 10% more that you can gain, but 90% that you can lose. So losing 50% or half of that 90% you already have would be far more than gaining 50% or half of the remaining 10% you don’t yet have.
90+(0.50*10) = 90+5 (50% of what you didn’t have is gained)= 95
90-(0.50*90) = 90-45 (50% of what you had is lost) = 45