My brain is stuck lol

Could a person use the BRRRR calculator in a seller finance situation with a balloon payment?

Would the BRRRR calculator give accurate info in that situation?

In the refinance section of the calculator you wouldn't be pulling money back out to reinvest (like the calc is intended) but to pay the balloon payment.... the numbers would work out the same ....no?

Yes, it will work the same. It is intended to do exactly what you want.... Purchase the property with a short term loan, then refinance into a longer term mortgage. Where the money comes from, be it hard money, private loan, or the seller, doesn't change the calculations.