Optimal level depends on your recording media. The best signal to noise is typically the hottest output you can get without clipping. The level you suggested for the MP20 metering is fine so long as you are not overdriving whatever is downstream from the mic pre. You may have to reduce the level on the MP20 to accommodate the next device in the signal path. To reiterate, the hottest signal without distortion is what you are trying for.

Always remember with instruments and voices that you are dealing with dynamic signals filled with transients (loud peaks). If you are not using a gain control device such as a compressor or limiter then you will need to adjust the MP20 to the loudest part of the signal to guard against distortion.