SharePoint List Calculation

Hello, I am looking for assistance please.  I have a sharepoint list for reimbursements.  Once the data comes through to my list I then manually select the date when I process the refund.  I am trying to create another column that highlights the reimbursement obligation once processed (0-3 months = 100%, 4-7 months = 75%, 8-12 months = 25% and after 12 months = 0%).  How can I do one column to show the repayment obligation percentage based on the time it was processed and then a separate column to show to calculations and sum from what is owed.  E.g. $800 was processed as a refund on 30th January 2024 so the Reimbursement Obligation will show it is in the 4-7 month (75%) category and then automatically calculate that the repayment amount at this stage is $600.  These percentages and amounts will change as time passes until eventually they will owe $0 after 12 months.  How do I do this?  Thank you!