I don't agree with this. For several reasons. One, there are variables. Like on one extreme if the OP paid cash for the house, should his friends stay there for free? No. And on the other extreme if the OP got into the house with little or no down payment on a 15 year loan then his mortgage might be high?(this is just theoretical). So therefore a third of the mortgage might be more than a fair or market rate.

Further, the friends have not put any money into a down payment. And they won't be asked, most likely, to pay for upkeep and repairs on the house. They shouldn't be asked for those things. It's part of the cost of ownership. But in lieu of that they need to pay a fair market rent or close to it.

So I suggest going with what is a fair rate. I can't speak to what that is in that area, so I will yield to others. But I have rented to friends before. I gave them a deal on the rent, but it wasn't representative of what my mortgage was because I was near one of the extremes I talked about.