with arcpy.da.updatecursor('TableName', ['FID', 'NewField'], sql_clause = ('None', 'Order By FID')) as uc:
    base = 40000
    counter = 10
    for row in uc:
        #Set the first value
        if row[0] == 1:
            row[1] = base
        #Set the value for odd rows
        elif row[0] % 2:
            row[1] = base - counter
            counter += 10
        #Only thing left is even rows
        else:
            row[1] = base + counter
        #commit the change
        uc.updateRow(row)