Stove pipe oval to round adapter question

I have an upland 207 wood stove. The outlet on the back of the stove is set up for 7" stove pipe that has been ovalized. After considerable effort I managed to form and fit the pipe into this opening nicely. Now I have to cut my pipe to transist into an elbow. What I'm trying to judge here, is how far back from the oval I have to go so that I'll still be able to make the other end round enough so the elbow will fit in it nicely. The pre-made adapters you can buy are about 12" long, so maybe that's a reasonable guess? I don't want to have this adapter any longer than I need to to, but at the same time don't want to cut it too short and have to refit the oval end with another piece. So just asking here in case anyone has done this before and can give any feedback. This is just plain uninsulated black stove pipe, it has a seam in it if that makes a difference.

Wasn't a big deal, I ended up cutting it about 11" long and it fit no problem, just as an update in case anyone else reads this.