I've got a interior stairwell with 20 steps of 7 1/2" rise on wood treads, except for the bottom step that's a concrete tread 6 1/2" up from a slab. The top of the stairwell features a pocket door with more or less a 3/8" sill (so technically that top step is 7 7/8"). Despite the somewhat uneven stringer cadence, it has been fine for many years.
But now we want to add a basement subfloor (dricore venting style) which adds at the very least 7/8". What can we do to even back up the stair treads? How bad is a short bottom step (I know it's better than a tall bottom step)? How thick a subfloor + carpet could be tolerated on the slab?
The current stair is carpeted, and we'll probably put back carpet. USA based California, home era 1920's.