]Alternative conversion process | Just a reminder]
A shortcut to manually convert a binary number into its two's complement is to start at the least significant bit (LSB), and copy all the zeros (working from LSB toward the most significant bit) until the first 1 is reached; then copy that 1, and flip all the remaining bits. This shortcut allows a person to convert a number to its two's complement without first forming its ones' complement. For example: the two's complement of "0011 1100" is "1100 0100", where the underlined digits were unchanged by the copying operation (while the rest of the digits were flipped).
In computer circuitry, this method is no faster than the "complement and add one" method; both methods require working sequentially from right to left, propagating logic changes. The method of complementing and adding one can be sped up by a standard carry look-ahead adder circuit; the alternative method can be sped up by a similar logic transformation.
source: Two's complement - Wikipedia
I said 48 because I have 48 inputs. at the moment only 24 piezos and 16 LEDs but I hope go to 48 piezos and perhaps buy a centipede shield and use 48 LED although at the moment twelve or 16 LED would be enough. Just extrapolating for the entire mux-shield;
Lol about the solenoid! 
guess I'm NOT 'doing the steps'
You have the centipede library so that removes one big problem for you, so stop worrying about the chip and how to drive it. You have that bit nailed.