Just purchased some Fischer Fx9's with the -12 binding. I understand how to adjust for boot size but what are the +/- 15 designations for on the side of the binding rail. What determines this setting? I'm a 250lb, 6'1", 45 yr old intermediate returning to sking after a 23 yr absence. Love green/blue groomers, working on my PMTS/carving instead of skidding. Any advice?