Your downthrust from the pilots view should actually be down-right and is usually 2 deg in each direction. Sometime a little more for a plane that has a higher power to weight ratio. Start by using the thinnest washers and work from there. Yes you can use shim stock to make your own washers. Really the best way to check for the right downthrust is to fly your plane. When in strait level flight, open up the throttle. If the plane rolls left, not enough right thrust. If it pitches up, not enough downthrust.