I think we get too hung up on the difference between stout and porter. Go back to Victorian times, and the only difference between the two was the amount of water used. You had porter, and stout porter, meaning that it had higher gravity. Pale ales and stout pale ales. Meh. We love to classify things, and create these artificial frameworks that let us put everything in their niche.
But there were no rules that if it had a higher gravity than a certain value, it must be a stout. It was whatever the brewer called it. Meaning some places had porters that were stronger than other breweries' stouts. Confusing? Not really, it's a dark beer. And it was whatever the brewer called it.
Fast forward to modern definitions. There's no requirement to use roasted barley in a stout. As @sneezles61 above mentions, the roasted barley is where you get the acrid/astringent character found in many stouts these days. But you can make a great porter or RIS from the classic combination of pale/amber/brown malt, with just enough black malt to give it the color you want. If you want to keep the astringency to a minimum, use the debittered stuff, like carafa III. Or sub the black malt for chocolate, with some crystal for sweetness on a base of pale malt.
Although I'm not a fan of the BJCP's method of classifying beer, I do think it's a good idea to use their style definitions for a starting point on OG, IBU, SRM, etc., as long as you don't get hung up on their overly restrictive metrics. It's still a good reference for where to start, so go middle of the road on their metrics for a porter or stout, and go from there.