As an aside, I'd suggest the pre-game discussion with your opponent about terrain as an excellent time to discuss what can vault where, and what can fit under where etc. (I'm personally still always happy to resolve that stuff in game as soon as the question arises, same as I would for any 'oops we didn't think of this when we discussed at game start' terrain issue - which includes slapping down sils if required to answer definitively - but ideally both players have as total knowledge of the table as possible before any models hit the board)
That's measuring a distance, though. However I think this is turning into an argument at cross-purposes. It's perfectly possible to give a player that's unfamiliar with a specific set of terrain information about what can fit where, without allowing arbitrary free placement of Silhouette templates. Apart from anything else, it's almost impossible to write a rule that gives an unambiguous threshold between checking which places a model can fit, and using the Silhouette Template to premeasure things.
I can see that placing a 25mm wide silhouette on a ledge could constitute a measurement of whether the ledge is more or less than 25mm wide. I suppose that checking whether a silhouette could fit through a gap could, in the same way, constitute a measurement of the distance between the sides of the gap. I wouldn't have considered that to be premeasurement, but you're the boss. So is this the rule?: We can check whether a silhouette would fit on or within a given piece of terrain, because we can measure the terrain outside the game and/or have played on it in prior games. We can't check whether a silhouette would fit in a gap between two terrain pieces, because that gap only exists once we set up the table, and we can't premeasure within the game.
You've just given an idea to Kickstart a line of "Optical Illusion" scenery, perfect for pranking your gaming buddies
