Yes - it's not 'my rule' really - it's what I've gathered from Hugh Piggott's ideas.

"Any other comments about the design and what you would do differently would be very much appreciated!"

The angle of the tail pivot could be changed.  It doesn't look like there is any side - what you have should work to protect the machine, but the power curve will likely drop off a lot in high winds after it starts furling.  Some side angle would be good and allow for more output in higher winds.   I also think the tail pivot looks very 'light' - I fear the tail might fall off after time.

The alternator spins on pillow block bearings - that seems fine, but I would be sure to put a spacer in there and not rely on the set screws themselves (You've probably allready done that).  I once got my fingers smashed pretty good by doing things that wway without a spacer.

I would go to 3 blades - it'll balance better.  No big deal to put a weight on it.  Machines with a pipe over pipe fit on the tower, and a furling tail are really inclined to shake/rattle with 2 (or 4) blades in my experience.  I've never done 4 - but I've done two by themselves - and two with a wincharger airbrake (which is a lot like having 4 blades).  The airbrake setup works well on a machine with a fixed tail, but not so well on machines with furling tails.

  "One thing I am thinking about how to add at the moment is a mechanical cable operated brake...  Dave B's post is interesting, but I would like to try for something simpler..."