I've never done a metal staircase, but the math would be the same. The first part of my plans for the wood version explain how to determine the tread widths and rises. That portion is there for the reading. I think a steel staircase would be easier, in that it's possible to weld situations in which fastening wood could be very difficult. The center pole on my design is steel, it just looks like wood because it's veneered. You might be able to adapt some of what I've done to what you need. Read over what I have up, if you have any problems, just let me know and I'll see if there's anything I can help with. It's too bad this board has HTML turned off, I'd set a link.