In the 24 plus years that I've been in this business, I can count the number of "Perfect" slates I've delt with on one hand. "New" slate, is just that, "New", which means, it hasn't been used before. Now, that don't make it perfect. ALL slates, to some extent need leveling, with the "Rare" exceptions I mentioned. Yes, slates are suppose to be ground to a certain tolerance, but, that's when it's on the grinding machine, not when it's on a pool table frame. Slates being flat over a large area, can and will flex, or lay to a certain extent, to the frame that it's being attached to. The leveling of the slate is more or less, matching it to the frame of the table. I can't tell you how many tables I've set up that the slate was perfect, but the frame of the table sagged in the middle by 3/4" under the weight of the slate laying on the frame. So, yes, it's important to be able to read them hills and valleys in the surface of the slate, side to side and end to end, in order to insure a perfect fit of the slate to the frame it's resting on, if you want to make sure that the balls don't roll off!!...LOL Slate WILL sag side to side under it's OWN weight, if it's not supported properly, and that's what you need a short level, to be able to see that sag. Think about it, a 4ft machinist level, would show you that from side to side, the table was flat, but because it bridges the slate entirely for the most part, you'd never see the sag in the middle of the level, because it's being bridged by the level. Yes, you could slide a dollar under the level in the middle and see that it don't touch, but that wouldn't tell you how much you need to raise the middle of the slate to raise up that valley to level. But, now an 8 inch level, that reads high on both sides, indicating that the middle was low, would also read level on both sides, once you raised the center to the same height as the outside edges.
Glen
