def total_calc(n):
     input = pd.read_csv(file_name)
     input['calc'] = 0.0
     for index, row in input.iterrows():

       # perform calculations

     input.at[index, 'calc'] = calc_value
     input.to_csv(file_name, index=False)

def total_calc(i,n):
     input = pd.read_csv(file_name)
     input['calc'] = 0.0
     for index, row in input.iterrows():

       # perform calculations

     input.at[index+i, 'calc'] = calc_value
     input.to_csv(file_name, index=False)

total_calc(1,2)
total_calc(2,8)