[s-cars] vac at idle

[WILLIAM HEALY]

Thanks to all who responded to my emissions issue.  I'm feeling good, that I'll beat the system next try. 
 
As a vacuum leak can cause an emission test failure. I thought I'd ask the guys who know...
 
What is a normal vacuum reading at idle.  On my autometer boost/vac guage I consistently get 15 inches of vac. What should I be getting?  I've heard 17 is normal.
 
Thanks again, look forward to your feedback.
 
Bill Healy
'93 S4