Hi Mark; Big Welcome to Permies! And a big welcome to the wonderful world of rocket science!
Congrats on choosing to use a rmh in your addition!
Are you thinking of a J tube?
I know that Luke sells the Walker J tube CF board stove.
I heard a rumor that he also has the batch
plans and could possibly produce one. (the cfb portion )
The batch rocket site spreadsheet that you found might be giving you btu numbers for a 6" batch / 8 " batch. Not a J tube.
I have lived with an 8" J tube and traditional piped mass since 2013.
I built a second 8" J tube in my shop last summer.
This summer I have converted the shop 8" J tube into a 7" batchbox and I am hoping to convert my other 8" j tube into a 6" batchbox before this next burning season gets started.
I loved my J tubes, but I love my batchbox even more.
Here's why...
Way more heat, lasts longer, wood is not split as small and it lays horizontal,fire burns unattended for over an hour... coals last longer, there is a door so no open flames....
I
think they speak for themselves!
So all that to say, I recommend a 6" batchbox .
A 6" j would need wood every 35 minutes or less, with multiple firings needed to get the mass up to heat.
I think you will be much happier with a batchbox design.