Possibly the logic of the fsm groove alignment spec is that it permits fine tuning the steering BOTH up and down. Raising the bridge rakes the steering a little improving the cruising stability, lowering the bridge quickens the steering and makes it a little more aggressive or sporty. The front and rear shocks, the rear brake pedal, the handlebars and even the clutch engagement permit fine tuning as well. Even though the fsm likely gives “a starting point” specification.