I have Ana's floating shelves plans from YHL for a 72" shelf. I want to build one much shorter, like maybe 2'. I noticed the frame front is 3 inches shorter than than the actual shelf. Is there a specific algorithm if I were to build a shorter shelf, or is the 3" a constant number? In other words, if I built one 24" long, would the frame then need to be 21"? And would the strips on the frame remain 11" apart?