It may be too late for you... Here's the solution I came up with too late for me to implement from another on this board on my chair rail on the steps project. The first piece of wainscoting (my spelling is closer to correct than yours ;-)) you put up at the bottom of the stairs goes straight, not at an angle. You only need about a 4 inch wide piece. Then you can split the angle coming down for your trim piece on top of it. then you are straight across at the bottom of the steps to make your outside miter.