How do I determine how much tuition credit I need to transfer to my parent to reduce their tax owing to 0?

You can either wait until they've finished their returns to see how much they owe (and which one owes the most), or you can make an estimate based on their previous year's tax returns.

If you have TurboTax Live Assist & Review, here is how to talk to a Tax Expert: https://turbotax.community.intuit.ca/community/navigating-turbotax/help/i-m-using-turbotax-live-assist-review-how-do-i-talk-to-a-tax-expert/00/873764