Rocket Stove Basics.

Hi All
Are there some basic starting points for a J Stove? I have been reading as much as I can, but still don't seem to know a fat lot.
I am building what I think will be a J stove, in that the fuel hopper has built up sides, rather than being fed in horizontally. Is that correct?
I am using the bricks from inside a storage heater to form the stove itself and then insulating around them. The cross section will be square rather than round, does that matter?
Is there a ratio of cross section to height of burn tube, fuel inlet length, etc, etc?
Thanks again for any help and advice.
Cheers
Nick

Go to www.rocketstoves.com and download Ianto Evans' book.