Installed correctly, the shortest distance from the DVL connector (in this case diagonal distance) to the nearest combustible (ceiling or adjacent exposed joists) needs to be 9" or more.
My stovepipe adapter is close to 18" diagonal from exposed joists / ceiling. If I recall, this is what I would have required according to Selkirk, had I used single wall pipe. With double wall this clearance is now serious overkill - but with the total sections of pipe I have, I played with the install until it gave me just under 5' extended above above the roof deck - so I did not have to install bracing, I still get a good draft, it's easy to get a brush down from the top, and I clear the nearest peak by just over 2'. That setup just happened to leave me with @ 9 or 10" of chimney extending down below the support box, which put the stove pipe adapter almost 18" diagonal from ceiling or exposed joists. I don't mind the look of the shiny chimney dropping into the room a bit - might paint it black one day - still undecided. But I have tons of room from adapter / connector pipe to anything wood. Which is a nice feeling when the stove is cranking. I have the ability to do this with this particular support box.
I like to place the stove a couple inches further out than required for minimum clearances. Besides the added peace of mind it gives you a little wiggle room for getting the connector pipe just perfect.
+1. As the king of do-overs, I'm finally getting it through my thick skull that "extra peace of mind" and "wiggle room" should be essential parts of any plan.
BTW - I added the roof bracing anyway this year after seeing another chimney get whacked by the weather. Not really a do-over, but....
Don't skimp on clearances, time, effort, etc.