Ask yourself this, how wide of an area is the shim placed in one spot, going to effect? Is it only right at the shim spot, or could that shim effect 3'ft left and right of that shim as well?I totally get that longer levels reduce knowing the truth. I also get that shorter levels can drive a person nuts.
If we're talking about brand new or well kept 20 year old tables, that's one thing, but what about an old T-Rail that didn't have levelers and machining wasn't 21st Century, maybe not even 20th.
Lastly, Is Slate that flexible I need to worry about avoiding pins.
Your description sounds like a leveler can telegraph its position and throw off a reading.