Gain staging

I just spent the last 3 hours reading threads pertaiining to gain staging. I've got a pretty good grasp, and actually realized through reading that I've been hitting my Motu 896HD too hot. I always record with my 896HD input in the yellow OOpps. Anyway, I use Sonar 8.5 which has a trim on each channel. So in theory, I want to adjust my trim so that my Master buss is at -18. But here is where I'm a bit confused. So say I have a vocal track, I adjust my trim so that my master fader is at -18. Now I add a compressor, after my VST compressor should the vocal still be at -18 on my master? I think I'm confusing myself here. LOL..

Or should I be looking at it as my compressor is say doing -1 db of gain reduction so I adjust my vst compressor output accordingly to get back to my roughly -18 on my master? Now, if I'm understanding correctly, as I add tracks all of which will be individually trimmed at -18 the sum of all of the tracks will increase my output on the master.

Gain staging