IF, the tile installation is approaching the walls "square" then it is a simple matter to measure from an imagined "tile point" straight back to the wall. Determine that dimension then transfer that dimension to your new piece of tile.
Use a speed square to draw your line.
To make it a little easier you could fashion a wasted corner and place it in the opening (pocket) adjusting for the desired grout line then measure from that tile point to the wall.
Remember to allow for a 1/4" gap at the wall.

If the layout IS NOT square then the same small piece of waste can be placed in the pocket and you then measure down both grout lines and transfer those dimensions to the new tile.
There are also gizmo's on the market to help transcribe the weird angles.
[Edited by Bud Cline Tile Contr on Thu, Jul 31, 2008 at 8:41 pm CST]