Due to doing a lot of meteor obs reports i got to know how to tell at a glance without even looking for specific stars roughly what the LM at Zenith is at my familiar sites - ie backyard ect. Slice i am surprised that you only see down to mag 5 at MB! In my backyard in suburbia near ttg plaza I can see down to nearly mag 6 in certain darker areas of my overall sky in the east and S/E - but in the north and west it is very bad - rarely more than 4.5 - 5. At our dark site near Mt Pleasant i have noted stars down to 7.2 at zenith (with effort) at times. Depends if you are talking about wether you can see them easily or with effort see them come in and out of view. I find the visual appearances of the Magellenic clouds to be great rough guide to conditions for us in the s.hemis.
I used to use this site
http://www.namnmeteors.org/lm_calc.html for meteor obs before I started printing out charts with snp to the mag i wanted and in the fields i wanted (I re-use them over and over forever now) - and then marking/numbering stars at various relevant mags and writing them down on a seperate page for reference.
Also the bortle scale is a great scale - wish it was more popular - I have read people can see down to mag 8 at extreme sites on top of 5000 mtr chilean mountains ect. in perfect conditions :-) wish there was a southern hemisphere specific version of the bortle scale.
http://www.frostydrew.org/observator...ays/bortle.htm
Kearn