Could have an opportunity or a situation ahead and want to make sure I understand.......

If I take out a large mortgage, one that is over the IRS limit of $750k for deducting interest, then interest remains deductible on $750k, but not the remainder, which could then be considered investment interest. So if I get a $900k mortgage, I then have the following equation to figure out what is deductible: 750/900=83.33%. So at tax time, I would multiply the interest shown on my 1098 form by 83.33% when itemizing. Am I right so far?

Interest on the remaining $150k could be considered investment interest, right?

Then later on, as the principal is paid down a bit, I would recalculate using the average amount of the loan over 12 months, say it's now $850k, so the new equation would become 750/850=88.24%, and I multiply that with the interest shown on the 1098. Correct?