Where is the best place to tap the line for the boost gauge on the STI?

same place as the WRX. INtake manifold.

The line at the BOV that a lot of people use is OK, but a bit far from the signal that you want to read, which is manifold pressure.
There is a little metal tube off the front, that points down, cut and tap the rubber line at it. Best, most reliable signal is the closest one you can get. Drilling and tapping directly is even better, but WAAAAAAAAAAAAY overboard for the .000000000000000000000000000000000000000001% difference it would make.

do it just like subaru does. off the bov line.
later
andy

the other even more difficult place is near the fuel pressure regulator ..but its a pain to get to
use the vac hose off the bov

if i remember correctly, the BOV vac line goes directly to the intake manifold (underneath the back runner). subaru taps their boost gauge upgrade there so i would always do the same (when possible).

i tapped in lower on that line more towards the manifold (also hides the T fitting for a cleaner looking install).

http://www.ridgewaybodies.com/misc/boostfittings2.jpg