If you want to distribute the stake fairly:
Divide the stake reward you receive by the total of the coins in the pool. Then you have the stake received per coin deposited. Then each user gets: (deposited amount * stake received per coin).
Or is this too simple?
Not everyone's balance is equal so this cant be done :/
Thats why you calculate the reward per coin deposited, then you can distribute reward depending on their balance. But I think I don't understand what you mean exactly.