Donnie,
Scanning level is basically a function of scanning speed.  Theoretically if you scan slowly enough you wouldn't need to increase the gain above your reference sensitivity level.  Most codes recognize that this is not how things work in the real world, so they require an increase in gain that should make most reflections full screen bangers so that you can easily see them while scanning.  Once you find an indication and determine it to be relevant (e.g. not geometry) you then dial back down to reference level to size and evaluate it.
Regarding the scanning level, this should be dictated by the procedure to which you'll be working (I presume ASNT gives you a procedure to follow when you do a Lev II ACCP test).  Beyond that if it's up to you to decide, I've always found 6, 12, and 14 dB to be handy depending on the situation (50%, 25%, 20% respectively).
Mankenberg