Modules & VBA :: Update Field Value Based On Another Field

I have the following fields in an A2010 form

PaymentMethod
Total
InstalmentAmounts
OurFee

InstalmentAmounts field is populated based on the results of PaymentMethod which is a combo box

Here is the current code

Select Case Me.Payment_Method.Value
Case "Three post dated Cheques"
'If three payments by cheque
Me.InstalmentAmounts.Value = Round(Me.Total.Value / 3, 2)

In some cases there will be a few pennies that the client "overpays" because the instalment amounts will not add up to the Total. I need to add up the TotalInstalmentPayments to find out what the client actually pays and then add the difference between TotalInstalmentPayments and Total to the OurFee field

So

Total = 500.00
Payment Method = 3 instalments so
IstalmentAmounts = 166.67 (rounded) so client pays 500.01 so the 0.01 needs to add to the OurFee field.