You don't determine rent. You do analysis and see what the market rent would be relative to the sales price to see if it is a logical purchase for a rental. I get nice rentals (not duplexes, etc) in nice neighborhoods and am able to get about 1.1% of total cost (sales price + refurb) in monthly rent.
So, for a house that takes $150k to buy, I need to get at least $1650 in rent. If I can't, I move on....