So cover comes in three flavors: (page 477)
Lesser Cover - A single character blocks your LOS to your target. +1 to AC, but has no impact on stealth.
Cover - an obstacle blocks your LOS to your target. +2 to AC and to Stealth (Hide and Sneak) checks.
Greater Cover - cover + the Take Cover action or GM Discretion. +4 to AC and Stealth (Hide and Sneak) checks, but only if it is greater cover by GM ruling, as the rule states you can't get the +4 greater cover bonus from cover + take cover because when you move you no longer are taking cover. (page 252)
So what rule of thumb are GMs using for greater cover?
I am trying to write a guide/flow chart for the stealth rules and greater cover is really undefined, yet matters greatly to sneaky characters.
It must be less than being completely out of sight (as you couldn't attack them), yet greater than cover (+2 to AC).
So what is greater cover? Of how are you ruling it?