Read more of the manual. 4 washers minimum, 8 washers maximum. 1 washer is the equivalent of one set of holes on the L bracket. Preferred method of install is level bars, so you set the L brackets for level bars and use washers to get the proper preload. 6-7 is what you will likely need.
HD truck or not the proper number of washers is crucial. Preload, tension, etc… whatever you want to call it is what transfers the weight. No, we don’t need it on these trucks but it sure does tow better with it. You bought it, why not set it up correctly?
When you combine front and rear it’s a lift. Front only is a level. It makes it easier for people to understand what you did, and help you, when you use proper terminology.
You did buy their level kit, but then you added to it. That changes things.