On Tue, 2 Sep 2003 13:54:42 -0700, someone wrote:

Get some cardboard. Fit a square into each angle formed by each step.
Tack them on to the existing stringer.
Tape them together.

All that cardbpard and taping is more trouble than it will help, IMHO,
and asking for error to creep in what with that taping together and
then having to take the taped together mess to the new work piece.
That much length of taped together cardboard will be quite flexible
even across the flat way that you are depending upon being rigid.

Its "just" a matter of measuring and laying out. And yes, I used to
be the layout man guiding a crew of union carpenters on complex
framing jobs. BTW, don't be too obsessed with following every minor
inaccuracy in the existing steps; layout the new stringer "right"
based on the rise & run, and then fit it, there sounds like there is
enough flex to take up the minor eighths that are probably floating
around in the job.

Stairs are not rocket science, they are rise & run, I have detailed
both wood and steel stairs for the shop to have the pieces pre-cut and
then put together on site.

MHO,
-v.