Of course a lot depends on the intened use of the bike- racing, touring, marriage wrecking, etc.
I've only been involved with building one tandem (for track racing) but I did a whole lot of research going into that project and I found a good general rule of thumb is to keep the boom tube at 70cm or a little longer. That allows for accomodating a wider range of riders while keeping a decent nose-to-butt distance for the stoker.