because all ski manufacturers measure differently and skis can be trimmed at the factory differently, a 168 to a 170 could be the same ski. i think you'll be absolutely fine with a 170. at the end of the day, take the 2cm difference and divide it in half, 1cm longer tail and tip, thats it. in fact, if you plan to centermount, even a 177 wouldnt be a bad thing if you are a strong skier.