I've run into this before, we each have our own way of handling it. I like to cut pieces of 1 x 3, or some type pieces I have left from another job that will fit between the chair rail and the floor on about a 30 degree angle. I tape the end touching the chair rail and the floor, hopefully it's carpet. I use liquid nails in six skips between the studs and nailings. Wedge the boards between the chair rails and floor for about 30 minutes and it's set.