Newbie ??? - Chimney height for a Harman 260????

What's up, guys? Newbie here. Looking at the Harman SF-260 (I know there backed up, but I wanna make sure I've answered all my questions before I order) I'm planning on putting it in my barn behind the house, actually in a boiler room, in an enclosed carport on the side of the barn. I haven't been able to find any info on how tall my chimney should be. I'm planning on going with a class A stainless chimney, and I'm aware of the "2 ft higher than anything 10 ft away" rule, but following that only brings my chimney height to 12 ft from the back of the stove. Doesn't really seem high enough. Anyone have any thoughts or advice? Thanks in advance.

I would recommend 20' above the appliance's exhaust port as a minimum.