Stock spring is too stiff for you. My son and I both tip the scales at 170# as well. I found that the 09-12 zx6 spring at 9.2 kg/mm allowed me to set both sags correctly. I'm starting with 35mm on my sons and 30mm on my bike. I did not count threads but spent a couple of hours measuring and adjusting. I'm starting with just two turns of compression and zero turns of rebound on both. You can find the zx6 shock on Ebay super cheap too like the GSXR stuff. I hope this helps.