spiral staircase

Hi. I have a problem. I'm trying to create a spiral staircase. The height is a fixed number (2,55m), the number of steps can change.

- define a point (center)

- move the point (lenght of step)

- create a line between these points

- create an arc between these points

- difine a surface by boundaries

- extrude surface. So i have the first step.

The problem is how to move the copies of this step in vertical direction, at different heights.

I tried this:

- create a polar series of my step, depending of numbers on steps. Now I have all the steps on plane z=0.

How can I lift the steps, with a single expression or function, so that each step has his correct height? 
In my mind, each step will lift:

- first step: 2,55/number of steps

- second step: 2,55/number of steps * 2

- third step: 2,55/number of steps * 3

Help me please. I can't do this.