Looking at the pattern, you do have to be very close to the row gauge to get the decreases in within the measurement. They start at 18" for the smallest size and should finish on or before 27". At 29 rows/4" that seems close. If your row gauge is slightly off, you may need to place the decreases every 3 rows or maybe even every other row. Measure your row gauge on the part you've already knit and then figure out how often you need to decrease to get down to the sts for your size [33 (35-42)]. Check also that you began the shaping at the correct measurement.
It seems to me that the pattern remains the same, knit across one row, KB1, P2 across the next except that you include the decreases.