Here is how to apply three numbers (Grid:Row x Column) to find a final coordinate.
Grid:
When you enter the lobby you will find multiple walls with post office boxes. A normal group of boxes is about 5 feet tall by 2 feet wide and a wall may have from one group of boxes to many groups of boxes. The "Grid" refers to all of the groups of boxes on any particular wall. Each wall of boxes is considered a separate Grid and there is only one Grid per wall.
To learn the cache coordinates you need to determine two numbers.
These numbers are the last 3 digits of two PO Box numbers. To locate the Grids which contain these boxes, look at the last 3 digits of the number of the P.O. Box in the upper right-hand corner of each Grid. Find the P.O. Box number that matches the desired Grid number (below) and you are standing in front of the Grid you need to use.
A Grid has rows of boxes that run from left to right and columns that run from top to bottom. The width and height of the boxes may vary.
Row:
Starting at the top of the Grid and counting from top to bottom, the top Row is 1 the next Row down is 2 and continuing on to the bottom of the Grid.
Column:
Counting from left to right, the leftmost box is in Column 1 and the next box from the left is Column 2 and continuing on to the right of the Grid.
All that is left is to plug in the last 3 digits of the P.O. Box number that is at that particular spot on the Grid into the cache coordinates below.
Due to numbers being skipped on the boxes, a small adjustment to the North co-ord is needed. Please subtract four from the North PO Box.
P.O. Box – Rocky Hill
To find the cache location, use the (Grid:Row x Column) format.
N 41° 40.(G481:R1 x C2)-4, W 72° 38.(G481:R1 x C20)