code: def get_CDRAT(L, sum_ptrm_checks, sum_abs_ptrm_checks):
      
      CDRAT = (old_div(sum_ptrm_checks, L)) * 100.
      CDRAT_prime = (old_div(sum_abs_ptrm_checks, L)) * 100.
      return CDRAT, CDRAT_prime
docs: input: best_fit line length, sum of ptrm check diffs,
          sum of absolute value of ptrm check diffs
      output: CDRAT (uses sum of diffs), CDRAT_prime (uses sum of absolute diffs)