Not sure if I follow what you are asking, but most plans come with some form of cut list. A few will list an approximate board foot usage. It's not really an exact science. You always should have extra lumber on hand because you will have waste, cutting around knots or cracks, you may make mistakes and need another piece, etc. I like to use: Fine Woodworking, Wood Magazine, The Woodsmith Shop, The Wood Whisperer, and Woodworking for Mere Mortals.