Pr0x... the pipe is actually calculated from valve seat to runner inlet.
take a string, and put it under an opened valve. mark it, fasten it by either closing the valve or taping it down, and then measure the proper length and see where you'd like it to go based on engine height in the engine bay.
20 inches from the valve seat is something like 14 inches, including throttle bodies, so in reality, the stack will be somewhere around 5- 8 inches.