I would expect roughly 30000 deductible (40000 * 750000/1050000). I refinanced this loan twice in 2020.
Some how, the refinance was treated as a separate loan so my loan principal is doubled (around 2 million) and I got the 40000 * (750000 / 2100000) ~= 14000. This significantly reduces my return.
Also, when computing the months for a loan, you should use 1098 box 11 (acquisition date) instead of box 3 (origination date).
