Jeff Wilder said:
No, we simply figured that if the wizard made the incredibly difficult Intelligence check, he calculated correctly. (Even with a +8, I needed to roll a 17. Which I did.)
As I think on it, it might have been better to make the DC lower (the math really is pretty basic), but make the roll in secret. Miss by more than 5, and the PC has successfully solved for "Ouch."
That's not quite what I was getting at. Given that the measurement was based on the Dwarf's sense of depth ("approximate depth" as given in the PH), and that (it seems) you did not know the layout of the room you were traveling to, I would have allowed the attempt, but the consequences would not be pretty. Guessing the distance is one thing; finding an open space "that" far away to transfer into, without actually seeing the destination, is asking the DM to roll on "the Wandering Damage Table" IMHO.
Incredible roll, EXACT positioning: Using that number, you try to land ON him. That's not allowed, and you all take damage. Being smart, you adjust by 5 feet in one direction or another. The DM consults the map; you either get lucky and pick a clear spot, or you pick a spot that is not clear. The result is still damage.
Average roll, APPROXIMATE positioning: If it is a small room, you are virtually guaranteed NOT to land in an open space. You all take damage.
Poor roll, BADLY OFF positioning: If there is a lot of solid earth/rock between you and the destination, and beyond it, again, you are virtually guaranteed to be landing INSIDE some of it. You all take damage. If not, that's why the DM has maps; arriving in mid-air over a large cavern is another possibility -- and you cannot act until your next turn, so no feather fall that round.
Note: The "Wandering Damage Table" was a joke reference in Dragon magazine many years ago, in an April Fool's column describing Killer DMs.