Should first/second/equity lines include rent-back PITI payments?

 Yesterday, I was working on a terms class I'm teaching in a few weeks and stumbled across some interesting facts on seller's rent-back and buyer's PITI (principal, interest, taxes and insurance) payments.

In defining what a buyer's PITI payment is on the California Purchase Agreement contract, it doesn't clarify whether that payment includes a second, seller financing and/or an equity line!

In discussing (arguing really!) with my Broker hubby, the fact that it does NOT clarify what specifically the PITI payment is, therefore it includes ALL loans the buyer has on the property, right?

When I contacted my favorite escrow officer, she told me that 99.9% of the time the PITI payment is calculated based upon the 1^st loan only!

Here's another interesting question that has come up on transaction or two in the past for me. If the buyer obtains an interest only loan, is the seller still obligated to pay the principal on the loan? What if the loan is a neg-amortization loan?

If we look at the potential cost of rent-back for the seller and incorporate all these loans into the mix, that PITI payment could become astronomical!

So my hubby Steve and I discussed (OK, argued again!) how to protect the seller from signing a blank check for rent-back based upon the buyer's PITI payment.

We came up with this;

"Seller to rent-back property for 4 days after close at the buyer's minimum PITI payment." OR "Seller to rent-back property for 4 days after close at no less than buyer's full interest only or minimum neg. am. loan payment."

Still, we have the problem of buyer's possibly switching loans during the pending sale transaction, and then we're back to writing a blank check! 

Is the best way to negotiate a seller rent-back to simply put in a per diem dollar amount that the seller is willing to pay and "fa-get-about" the PITI payment?

How do you define PITI payments in your area?