Hello David
     I have had a vacuum guage in my '86 syncro for about 4yrs now.  I
decided to run a sereate hose.  I did'nt know if there were other devices
in-line with the hose to the brake.  I have been getting about the same
readings that you stated in your post.  Idle is about 14 " of Hg and cruise
is about 10 "of Hg.  Full throttle will not yield O vacuum.  There will
always be a vacuum, otherwise air would not be drawn into the engine.  The
erratic fluctuations of the needle could indicate leaky valves.  I have read
that one of the tools used to analize intake funtion is a vacuum gauge and
the book stated that a fluttering reading indicated a valve was not sealing
and allowing pressure back into the intake system.  By the way vacuum is
usually measured in inches of mercury or inches of water.  Most automotive
guages use the Inches of Mercury  " of Hg.  
     I use my vacuum guage to check for changes in the engine performance.
If I start to loose vacuum at cruise it usually indicates a t;une up is due.
A vacuum guage is a good indicator of overall engine efficiency once  you
have built up a history of what conditions give what readings.  Good luck  I
think you will be pleased with the guage.

Colin Moran
'86 syncro GL  168,000 and still going strong
Colin A Moran

K&M Ranch