ARCPY using the arcpy.da.UpdateCursor

[CornJulio]

Howdy,

I am trying to get this script to create Grid Numbers for me in a mapping fishnet,  but I am running into troubl with the error listed below  - any suggestions ?

import arcpy

arcpy.AddField_management("N:\GIS\Projects\Steve_Eastwood_Landuse\Plant_Biosecurity_Project\A0_Maps\Ao_template_Maps.gdb\Data\NTLLS_Fishnet" ,"REF","TEXT","","","5")
rowCount = 0
with arcpy.da.UpdateCursor("N:\GIS\Projects\Steve_Eastwood_Landuse\Plant_Biosecurity_Project\A0_Maps\Ao_template_Maps.gdb\Data\NTLLS_Fishnet",["OID","REF"]) as cursor:
    for row in cursor:
        leftPart = (rowCount % 4) + 1
        rightPart = ["A","B","C"][row[0] % 4]
        row[1] = str(leftPart)+str(rightPart)
        cursor.updateRow(row)


        if ((row[0]) % 64) == 63:
            rowCount = rowCount + 1

Error:
Traceback (most recent call last):
  File "C:\Users\hawkinle\Desktop\STDTAS\Fishnet2.py", line 8, in <module>
    rightPart = ["A","B","C"][row[0] % 4]
IndexError: list index out of range
[Finished in 14.8s]

Thanks Leith