Check this link for a really good dissertation about boost control...
http://webspace.webring.com/people/mo/ozbrick/engine-turbo-keithspage.html

There's a good diagram on the Matthew's site that comes up if you google "Volvo 850 vacuum" that shows the vacuum lines. Of course on a turbo, the vacuum lines change over to boost lines as the turbo spools up....

The shortest path appars to be from the nipple on the underside of the compressor side of the turbo right up and around to the wastegate nipple...the controller would be located above the turbo next to the ABS unit...

I'm thinking about putting an MBC on my '94 and have been considering Keith's recommendation of using two, one between the ecu controlled soleniod valve and another to control spikes connected between the manifold and the wastegate....haven't taken the plunge yet tho...