for racing here we are not allowed to run less than 6mm. which is about 1/4 inch, i think you can go as thin as 3/16 but then you should propably run two vertican supports (nascar style) to prevent it wobling, but a windscreen with high curvature will be less prone to wobble than a flatter one. and when it gets scratched (which happens fast) you can buff it with a paint buffing and polishing compound, just dont use the coarse stuff.