Spiff Tool Commission Calculation Setup

I need assistance with configuring the Spiff tool within Salesforce to accurately calculate commissions based on payments and invoice data associated with Opportunities. Specifically, I have invoice and payments data where payments can be made partially for an invoice, resulting in multiple payments per invoice. The task involves ensuring payments data reflects correctly on the invoice details by Opportunity, then calculating the commission percentage for each payment according to its payment date, and finally rolling up total commissions due per agent.

Here's how the setup should work:
1. For errors or missing data in records, calculations should proceed by using default values.
2. The commission percentage will be fixed across all invoices.
3. The same fixed percentage will apply to all agents.

Let me know how you can assist in implementing this setup within the Spiff and Salesforce environment.