Do you definitely want the 18 blocks to run sequentially? If so then you could use concentric loops where outer_loops has nReps = 18 and the inner trials loop has selected rows $useRows. Then add a code component in a routine that is in the outer_loop but before the trials loop starts with the following code in Begin Routine
useRows = str(outer_loop.thisN * 8) + ":" + str(outer_loop.thisN * 8 + 8)