Where your threshold is set will depend on a few different things. Whether or not you use headphones is one. And even if you do use headphones, the impedence is another factor, as well as ambient noise level, external noise pitch, the style & fit of your phones, and the ability of your phones to block or cancel noise. Add to that varying degrees of hearing loss, the speciffic frequencies affected by the loss, and personal preference, and it becomes easy to understand that there can be quite a range of threshold settings that could be suitable.
Personally, I run with Auvio wireless headphones, and my threshold is normally either 1 or 2 on the Xterra depending on external noise. And I have considerable hearing loss as well. Gunfire, racing engines, wind noise in my helmet, and incredibly loud rock & roll have all wreaked havoc on my hearing, and as a result there are certain frequency ranges that I cannot hear at all regardless of their volume. I am fortunate that most of the lost frequencies are on the high end, so it doesn't impact my detecting too much. But when it comes to alarm clocks and alarms on watches, coffee makers, microwave ovens, etc... for the most part I can forget it....to me they don't exist.