Stretching it shouldn't change the gearing, you should still run the same as your stock arm unless you've "added" power somehow. I'm running a +2 and I was running 14-39 with 18" tires. When I put the 21" paddles on I couldn't pull top gear at Silver Lake so I dropped the front sprocket back to a 13. I gave up some overall speed with the 18"s on but we mainly ride woods (like Rose City) and the slow crawling ability was improved.
Ross..
My Rides
1985 250R original (Garage Queen)
-under restoration
1985 250R black and blue full rebuild ( Daily Rider)
-Paul Turner pipe
-+2" Westcoast
1985 Red and White Tecat-EH Sold
1985 Green Tecate Sold
My Wife's Ride
-2003 Suzuki LTZ 400 Showroom fresh and completely stock
My Kids Ride
-2004 Yamaha Blaster, Completely Stock
-2006 Yamaha Blaster, Completely Stock