@Deborah Hickman Usually, when dealing with real estate loans/private money loans, the interest rate quoted is an Annual Percentage Rate (APR). So a private money lender loaning you $250k at 10% interest, is most often loaning you that money at 10% APR (as opposed to a fixed return on their money whether you have it for one month or 12 months).
If that's also the case here, then no - you would not be calculating this correctly.
Most private money loans are interest-only (as opposed to fully amortizing like your home mortgage loan probably is), so let's assume that this is too for this example. If you were to borrow $250k at 10% interest (APR) from your private lender, that would equate to a monthly interest-only payment of $2,083.33 ($250k x .10 = $25k divided by 12 months).
Now, if I was your lender, I would require you to pay me that payment every month. However, let's say for the sake of your hypothetical that your lender didn't require monthly payments and allowed you to just pay them when you sold the flip. Whether it takes you 3 months, or 6 months, you're going to owe them $2,083.33 per month because that's how much 10% interest (APR) equates to. Make sense?
Of course, you should confirm this with your private lender and it should all be documented and spelled out in the terms of the note. However, this is how it's normally done.
For what it's worth, and so you're aware, doing it the way you proposed may not even be legal under some circumstances and in some states. For example, in my state, anything over 10% interest on an ANNUAL basis (APR) is usurious (illegal) unless I go through a broker or there's some other limited exemption that the loan qualifies under.
So, if I personally negotiated/originated a $250K loan as the lender (I'm not a broker) for 10%, and it took the borrower a full year to pay me off (and I only collected a year's worth of interest), there would be no problem with that. However, if the borrower paid me off in only 3 months, but paid me the full year's worth of interest ($25k) in that short time, that would likely be an illegal loan in my state and all the risk is on me as the lender. (Since the interest rate as an APR would then actually be well over 10% APR.)
Anyway, just something for you to be aware of. Hopefully all that wasn't too complicated and made sense.