hey there--

I've spent a fair amount of time doing exactly the same thing you are.
In my experience, setting the PT outputs to -18 nearly always results
in console overload at the input. -15 makes a bit more sense in
general; with older SSL's I've sometimes had to go all the way to 0VU=
-12dBFS to avoid having channels break up on me before I've even done
anything. This will also enable you to run more faders closer to
unity, where their resolution is greatest.

By the way, lowering those outputs doesn't reduce your headroom at the
console--at mix, it just avoids overload at the input. At the same
time, in my experience it's hard to get enough output from most analog
consoles to drive digital systems to 0dBFS without distortion when 0VU
is set to -18dBFS.

Hope this helps,

Gordon Rice