re: When does it make sense, mathematically, to accelerate mortgage payments?

When it is your highest interest debt and any other allocation of capital cannot generate a higher return than the interest expense.

I also factor in the risk. The mortgage paydown is guaranteed return. So let's say you have a 7% mortgage and you are comparing that to a 10% return from another use of capital. The 7% is guaranteed. So that also plays into my decision.