Transaction table logic for declines, refunds, split payments

Transaction table logic for declines, refunds, split payments

I have a transaction table - with lookup columns for transaction type (approved, declined, refund) - approved need to leave the "amount" field as is, declined need to zero out the amount field, refunds need to make the amount field negative.  I have another lookup column for split transactions - the 2 split records need to be counted together as a single transaction.  Where do I start figuring this out?