How to balance a slot machine

[Atari]

Hi, and Merry Christmas.

I have a question, concerning the balance of a slot machine.

I have been working this last year, on a little slot game for my self, but i can't figure how to balance it.
I am playing 3 small systems, that each has got a certain payout percentage.
So i multiply these systems.

At the end, i am running a little lottery, that is based on the amount that can be won, in respect to a prize.
So, for example with the amount of 2, i add 20 %, and it becomes 1:2.4 .

I like to have it pay out with a real balance, which is for example 80 %.

I don't know how to calculate this.

Can someone help ?