Asked for the Boost pressure to be checked at the 5000 mile service on MY05 STI PPP and told by dealer it was spot on the spec. Also had a Boost gauge fitted the same day and can report my readings:-

Tickover: -20 PSI (-15 PSI when cold) stange, but whatever !
Decellerating: -25 PSI
Full throttle peak 18 PSI dropping back to 15 PSI all the way to the red line.

Does this sound about right and how accurate is my gauge ?

 

sounds about right. the turbo isn't working when you're not on the gas so what the gauge reports is the vacuum caused by engine sucking in air. obviously it pulls in more at higher revs which is why you get more vac at revs.