I’m building a house and the builder has a number of incentives you can choose between. It led to me making my own (semi-vibe-coded) web app to compare different options and try out different combos (“What if I applied this ‘flex’ money to the house price or if used it to buy down the rate”). It was even easy to wire up Firebase for saving the data so it would sync between my phone and laptop.

One thing missing from this calculator (which doesn’t technically matter for calculating how much faster you’d pay down the mortgage) are HOA, taxes, and insurance. Again, that only matters if you want to calculate the total out of pocket monthly.