Loosen the tensioner a bit and install the new belt. Then rotate the crank a few times and the tensioner will automatically set the tension for you. Tighten the tensioner bolt to the recommended torque spec, and turn the crank to inspect belt tension. It's possible that you have too many teeth on the belt in between the cam gears, and you'll have to manually turn the gears while putting on the belt in order to get it right.