In article ,
cj wrote:

i have a wall that is about hip high that forms a 90 degree enclosing a
set of steps going down to the basement. i would like to widen( or cap )
the top with some wider lumber so that it may be used for placing
knick-knacks, pictures etc. but it seems to me that the length of each
piece (forms a 90 )needs to be longer than the narrower mitered pieces
that i will be attaching to. am i right and is there a formula to
determine what the length of the new wider pieces of lumber that will be
attached?
thanks, cj

Forgot your junior high trigonometry? It's a right triangle, fer
christ's sake. If the overhang is 3", it'll be 3" longer as well.