I believe the front of the deck should be a little higher. This allows air flow under the deck. The blades do cut twice higher in the front, your selected height in the rear. To have a good vacuum you have to allow air in. Speed is important depending on conditions. Blades overlap in straight forward cut only. Turning left or right blades they do not overlap. The front blade should not be more than 1/4 inch higher, perfect at 1/8 inch. Blade rotation in traveling forward, half the blade is traveling backwards.