We switched to North main and genoa last year and use their general tuning advice. I see what I believe to be significant back winding on the main caused by the leech of the genoa. Some call it a 'speed bubble' and have advised us to ease the genoa to six inches or more off the spreader. We typically trim to 2-3" and set the jib car so the sail breaks evenly. The backwind is noticeable in all conditions and tune.
Originally I thought the problem could be solved by inducing more pre-bend with tune or backstay to flatten the luff curve of the main. But that doesn't really seem to help and sets us much tighter on the rig than advised.
