Kindle Unlimited scammers on Amazon
It should be divided by user. 
Let say the service costs 10 usd per month. So lets say you are going to pay 5 usd to authors. You should divide each user 5 dollars between the percentage the user read of each autor, so if a user only read one author, then the author gets 5 usd. And if he reads 80% of one and 20% of other authors, then the same percentage should be applied from the 5 usd. 
I do not know how hard would be this to implement.