Nick
Not only will the width and #of strands effect the dia. But the thickness of the strands will play a role in the finished dia. I'm real guilty of not writing stuff down when I do it. Either I forget too or more likely I'm just to lazy. And since I dont braid alot of reins or cores for them I always use the figure it out as I go along method when I do build them. The good thing about a rein rounder is I can braid the cores bigger than needed and just keep running it through the rounder until I reach the desired dia. I know this doesn't answer your question but it's how I go about doing it. Hopefully someone will come along with a better answer.
Best of luck, Mike