Mrf-13
I have the same windshield and live on a gravel road. I feel your pain with the dust on the shield.
I've been using the same method as Phantom suggested but was able to find a product at Ace Hardware called Plexi-Clean for under $5 a bottle(pint).
It also has a anti-static feature to it.
For anyone else lucky enough to live on a gravel road this also works great on the back panels on the bike to keep the dust from clinging to the back of your freshly washed bike.