Billing Schedule / Payment Plan Set-up

Has anyone figured out a way (other than manual) to have Payment Schedules calculate based on balance due rather than on "total contribution, invoice, or order amount" as per the documentation?

We're trying to create payment plans that calculate the following based on the balance due:

• 4 equal payments (25% each of the order total less the deposit amount)
• 9 equal payments (11.11% each of the order total less the deposit amount)

My thought is that there are often situations like this for orders (or for pledges where someone has gotten behind on payments) and all the manual work to "fix" the payment schedule is going to lead to frustration and errors. We're already noticing on the 4 payment schedule (based on the current rules) that the fourth payment is often left off because the deposit is higher than 25% of the order total. We're not using these payment plans online currently, but I could see us wanting to do that in the future as well.

Should this be an enhancement request to the way the TR_BILLING_SCHEDULE works? Am I'm missing something obvious?

Thanks!

Sara