Here are the names I've given to the parts in the dome deck structure:

Riser

There are five risers in the dome deck. They are the short, vertical struts that go up from each of the five verticies in the pentagon that forms the top of the dome itself. These should be made from 3/4" conduit.

I layed a length of uncut EMT across the top pentagon of my dome and measured the distance between the bottom of that EMT and the top of the dome. I foolishly figured that that would be the length of the riser. I was wrong, but, as you will see later, it turned out not to be a tragic mistake. What I failed to account for was that the angle-brackets add some length to the risers. I made my risers 14.5" high (which meant that I cut them at 16.5" so there would be 1" from the end of the riser strut to the center of the hole I would drill in each end.). I ended up needing to add a spacer of about 2" on top of the center vertex of the dome so that the radial deck supports would be exactly horizontal. So, I really should have cut my risers at 14.5" for an effective height of 12.5".

Deck Pentagon

The deck pentagon is the same size as the pentagon at the top of the dome. It is connected to the dome's top pentagon by the risers. The length of these struts are the same as the length of the top pentagon of the dome. You'll need five of these. They should be made of the same size conduit as your dome (presumably you have used 3/4" conduit for your dome).

Deck Diagonals

The deck diagonals connect the bottom of one riser to the top of the next. You must be certain that they are all "bottom-to-top", so that two deck diagonals never meet at the same bottom or top of a riser. I calculated the length of the deck diagonals using the pythagorean theorem. My top pentagon struts were 61" hole-center-to-hole-center, and my risers were 14.5" hole-center-to-hole-center, so my diagonals were 62 11/16". These should be made from 3/4" conduit.

Radial Deck Supports

The top of each riser is connected to the center bolt of the dome with a radial deck support strut. These end up supporting the deck as well as making the deck structure rigid. I actually assembled the rest of the dome deck structure without the radial supports, and then measured how long they needed to be. Mine are 52" long. These should be made from 3/4" conduit.