Controlling label/attributes of grid/fishnet index using ArcGIS for Desktop and ArcPy?

I have used the Grid index feature tool to create an index grid. The label/attribution of the page names are populated in a "row" orientated manner. eg lef to right a1,a2,a3,a4,a5,a6 b1,b2,b3,b4,b5,b6 c1,c2,c3,c4,c5,c6

How can I calculate a field so I can have the page names label/attribution in a "column" orientation?eg: a1,b1,c1,d1,e1 a2,b2,c2,d2,e2 a3,b3,c3,d3,e3 a4,b4,c4,d4,e4

Answer

Try this field calculator (Python) on a new text field:

def CN(nCols,nRows,j):
 nR=divmod(j,nCols)[0]
 nC=divmod(j-nR*nCols,nRows)[1]
 theDiv = divmod(nC,26); SecondL = theDiv[1]
 FirstL = theDiv[0]; aLetter = chr(65 + FirstL) + chr(65 + SecondL)

 aLabel = aLetter + str(nR).zfill(2)
 return aLabel

CN(42,47, !PageNumber! )

If you want to see this: