How does a player determine how far away something is to d-door to it/inside it?
For example, there is a building with a bunch of guards patrolling outside it and the player wants to d-door inside the building to surprise its occupants. The player has not been inside the building, another party member has seen inside.
Let's say it looks about 500 feet away, but the player is actually standing 505 feet away from the outer wall, which is 5 foot thick and the party of 4 is standing in a 10 foot square formation with the caster in a front square. The building is 40 foot on each exterior side. My understanding of the rules is that if the party wants to safely d-door into the building, the caster would need to know that the distance is at least 520 feet, but not greater than the far wall at 545 feet.
How would the player be able to determine the specific, straight line, distance to cast? Is that building 500 feet away? 600? 650?
My players are wanting to use Dimension Door to sneak around and repeatedly surprise an enemy encampment. Dropping in from outside the perimeter walls to wreak havoc.
Teleport at least gives a percentage chance of a mishap. How to determine if they land inside the building or outside when the exact distance can not be known?