Adding Non-Deductible Amounts to Contributions

We'd really like to have an added field to contributions entry, that would allow us to enter a non-deductible amount (since we're legally required to track this).

Ideally, this field would do some automatic math with the contribution amount and also track a total of deductible amount.

For example:

Contribution amount: enter $100
Non-deductible amount: enter $25
Deductible amount (automatically calculated): $75

This field would generally subtract zero from the contribution unless filled in otherwise.

Adding this information into the notes section of a contribution is clunky, and not always easy to notice at a glance, and makes reconciliation with our finance director a bit trickier.