What do the step size table and step size adaption tables look like for a 12 bit input conversion with ADPCM?

Question

I'm following this guide on using ADPCM to encode 12-bit ADC values into 4-bit samples on an MSP430. It works well as-is, but it's designed to handle 16-bit ADC readings, so I am losing resolution with the current algorithm. I'm wondering how the following tables would change for a 12-bit input signal instead.

// Index changes
const signed char StepSizeAdaption[8] = {
    -1, -1, -1, -1, 2, 4, 6, 8
};

// Quantizer step size lookup table
const int StepSize[89] = {
       7,    8,    9,   10,   11,   12,   13,   14,
      16,   17,   19,   21,   23,   25,   28,   31,
      34,   37,   41,   45,   50,   55,   60,   66,
      73,   80,   88,   97,  107,  118,  130,  143,
     157,  173,  190,  209,  230,  253,  279,  307,
     337,  371,  408,  449,  494,  544,  598,  658,
     724,  796,  876,  963, 1060, 1166, 1282, 1411,
    1552, 1707, 1878, 2066, 2272, 2499, 2749, 3024,
    3327, 3660, 4026, 4428, 4871, 5358, 5894, 6484,
    7132, 7845, 8630, 9493,10442,11487,12635,13899,
   15289,16818,18500,20350,22385,24623,27086,29794,
   32767
};

Since I am also handling the decoding, the tables don't have to conform to any standard, but I would like to follow one if it exists for 12-bit values.