A common height for wainscotting is approximately 42", though it can be found in a wide range of heights. Its traditional function was to protect the lower part of the wall from abuse by furniture, and to serve as a backdrop to complement furniture. 32"-34" wouldn't serve these functions and would, I think, look awkwardly low. BUT, there are practical considerations too. Set it where you like the look, after also considering what furniture will be in the room, window locations,avoiding light switches, etc. As far as economic use of 96" material, 1/3 of a sheet =32", plus 6-8" base plus 2-4" rail gets you to approx. 42". The apron would be a separate feature.