Is there a reason for the rectangle, or is that the idea just for simplicity? Like, "smallest convex polygon" might be slightly better. I don't see any reason why the borders would need to be rectangular. Also for any shape there's no reason it would need to be axis-aligned with the exterior perimeter (which itself might be far from rectangular.)
It's a very simple rule of thumb that someone can check out in drafting software (or even paper blueprints). It's applicable in one-story domiciles (apartments, bungalows) and multi-story homes.
Gary Klein, the fellow who thought of it, has been consulting on (hot) water issues for a few decades now, and so has tried to whittle down his advice to the simplest thing that (a) people will understand, (b) be actually implementable. That's generally is: make all the hot faucets as close to the hot water source as possible. The rule is a metric for that.
Hah, this reminded me of a submission from a couple of years back [0] about efficient hot water piping, and lo and behold, the URL is for Gary Klein Associates [1].
