Here's Sonic's suggested spring rate table:
http://sonicsprings.com/catalog/calc...pring_rate.php
My 110 lb. daughter rides it more than I do, so I chose the springs as a compromise for both of us. I got 0.70 kg/mm springs. Filled with oil to Sonic's suggested level, they are plenty stiff for me. I can do a stoppie on bumpy pavement an they don't bottom out. I don't remember the % static sag I went for, but I usually try for abut 20%. Of course you adjust that with the spacer length. They supply PVC pipe for making spacers.