I am a third gen. mason who has been building Orton style slanted Rumfords ever since I read his book back in 70's"The Forgotten Art of Building a Good Fireplace" I have 3 at my home of which through years of experimenting and observation I have acheived a 1:30 throat to fireplace opening ratio. This is a 50% reduction in throat area over any type of fireplace out there that I know of including the straight back so called " True Rumfords" that Jim Buckley advocates and sells throats for at his online site, Buckley Rumford Fireplaces. He has seen videos I've sent him and is awe although he remains skeptical while wanting to know if I could give him more vivid videos and details to my secret and if I could make his do similar ratios!
He said he may want to buy or market my discovered fireplace secret !