If you want to see how someone on this Forum built their helixes, click here. This is Big Boy 4004's Dream/Nightmare thread. You need to go way back to some where in the first 10 pages or so. (The thread is currently 50 pages.) Quite insightful. When I saw what he did, I decided not to build a double layer layout. The helixes do use a lot of real estate! If you go with O72, plan on using roughly 64 square ft. (8x8) 6x6 does not allow for overhang!
Chris
LVHR