I recently built a 200 gpd system for myself. It started out at 2.5 gal flush/ gal pure, but I recently modified it and am getting about the same 1.5:1 ratio, relying on line pressure only. PPM in = 210, RO = 10.
The simple fact is that lowering the flush water ratio must increase the fouling rate of the membrane, but David's experience is most encouraging.