You have a mixed use loan consisting of both acquisition and non-acquisition (equity) debt. In accordance with Pub 936, principal payments are applied to the non-acquisition balance first until it is paid down before applying any principal to the acquisition balance. The ratio of the acquisition balance over the total mortgage balance increases from 75% to 100% as the non-acquisition balance is paid down.
The amount in the 'Since you first took out this loan' box is always the total amount spent to build, buy, or substantially improve the home associated with the original mortgage and any refinance. In your case, it remains constant at $300,000.
