Or at least that is what I swap around between depending on the site. If the pops + clicks bother you, then you can run 40 disc to be about as silent as you can get as your not in that "boost" range. So, when I get up around 40 on the disc, I'll increase the sensitivity into the 90's (really my T2 never goes out of the 90's). Running around 21 will still get ya lots of pops & clicks, but not like under 5 will.....
I believe it's under 4 is where you need to be for that "power boost" by the way. I reserve the 211 (21 disc/ 1 tone) method for hunting in nails, as in a bed of nails you don't need a power boost......
I guess it all equates to how deep you need to go in that field? If things are plowed up in the field 40 may work for you. A fourth option would be all-metal mode as it sounds alot different (smoother?) than the disc boost area, but then your hearing more than a 21 disc even....
I think of it this way.....you need top depth you gutta hear some chatter. With very few virgin sites left here, I have to get as deep as I can, so I have been running with basically a "threshold" so to speak with disc under 3, & sensitivity in the high 90's. Annoys some people, but I just turn the volume way down in my headset & watch numbers, & lots of hunted out sites are producing again with this.....
HH,
Bill