Andy,
The best distance between stacked beams is the most you can get and
stay sort of practical. If the mast between beams is too long then
it's going to bend, which is bad. If too short, then those beams
might as well be all on the same boom. And until the distance between
them gets really ridiculous, they are going to affect each other no
matter what you do. So, I guess it depends on what you can manage.
There are probably as many 'rules of thumb's as there are thumbs
putting up stacked antennas, so, pick one you can live with.
- 'Doc