One of the best innovators on RMHs (Kirk Mobert aka Donkey) suggests that the best way to place the barrel over the riser is to fire up the stove without a barrel over top, get it going good, then lower your barrel over the riser listening for the "sweet spot." You will notice that the rocketiness of the stove increases to a roar as you lower the barrel, then quiets down again as you get too close to the riser. The idea is to find the spot at which the roar is loudest, which means going through it up and down a few times, then backing it off the most roaring spot (up away from the riser) so that it's a little quieter, but still rocketing. Kind of hard to explain in words, but if you try it you'll catch on pretty quick. Of-course if you use this method, be sure to have some really good gloves on so as not to burn your hands...and a buddy to help you place supports underneath the barrel as soon as you get it where you want.
I think the principles at work with this method have to do with finding the most efficient placement for your stove, as each stove is unique...formulas give a good general idea, but i've found this to be the best way to find the right placement. Too much rocketiness means the gasses are being pushed through too fast and not burning super clean.
Curious if you decided on a rmh for the tiny house after all?