I forget MDF is not nice to machine, Being an old git and growing up in workshops without dust extraction etc. I have become used to it, Iroko is the one to try, still makes me cough for the first day, guess that's why they call it Ichokeo. Here is a very quick pic to show where the margin piece and pitch board are used, you can see once the treads and risers are marked the nosing and the stair jig will sit into the margin piece area. This picture is to scale for your treads and risers, I have drawn the stringers at 275mm which is a standard size for stringer stuff, most joinery shops will use southern yellow pine for stringer stuff as it is very stable but a bit more expensive than Russian redwood; which is more commonly used for the treads. I have also attached a picture of the underside of a staircase to show the amount of material behind the tread and riser. the wall string shows how yours will look the second image is the cut string to the well side, you can see how the cut string has no wedges and the bulk of the stringer is set under the treads. This is not a staircase I made so sorry about the quality of work but it gives you an idea. I will go through all this in more depth to guide through.