you need to manipulate pos, which runs as a fraction of the table at 0..64
so as its 0 to 64 and you have 128 cycles in the whole table, 0.5 = 1 wave.
so the start is wave number * 0.5 and you need to run for 0.5 before go back to the beginning.
if you have a phasor running 0 to 1 (check its not bipolar!) then you need to multiple that by 0.5 to to run.
e.g.
wave 32, starts at 16, and runs to 16.5
of course if your waves don't EXACTLY fit then you will need to adjust the maths left to the reader
and also if its not 128 waves, they you have to do NWaves /64 rather than use 0.5
of course multiplying by 0.5 is the same as binary shift right (>>1) which is more efficient.