You may not like this reply, but here goes:
Make them any spacing you want. The SteppIr 2-element beam had 2 full size elements on a 5 1/2' boom! Yep, 5.5'!! Did it work? Sure did!
My question would be, "How long is your boom?" If its 12', then space them 12'. If it is 11' 5 1/4" then space tham 11' 5 1/4"! Yes, all of these dimensions work.
My second question would be, "What are you trying to accomplish with this antenna?" Are you looking for the maximum amount of gain with 2-elements; are you looking for a maximum front to back; what kind of bandwidth do you need? So many questions!
My final question would be, "Why do you want to use a driven/driver rather than a reflector/driven?
Figure out what you want and let us know. Maybe we can help you. Be prepared to tell us the diameters and exposed lengths of the tubing, the size of the boom, whether you are grounding the director to the boom, the size of the brackets, and what type of feed point you plan on using. At this point I'm assuming an insulated split driven element with a choke balun as a feedpoint, but I can be wrong.
