As I've said on other threads, all a monitor will do is beep at a number, whether you're good to go or not is something totally different. I know somewhere a lot of people go which (on the flat) varies between about 11% and 14% O2 and people regularly strut up there. I've been on a rope very carefully in 15% and it was fine.
Generally, I obey the "lighter won't light, get out" school of thinking.
Read this for starters:-
http://wasg.iinet.net.au/Co2paper.html I've had 3 4 gas meters and really, the LEL is pretty much irrelevant in metal mines, H2S is soluble and shouldn't really present itself in abandoned mines and none of them detect CO2. Low oxygen is your major concern.
I find a Davy lamp a good tool, but if you fuel it wrongly it goes out too soon and it won't detect the difference between low O2 and CO2 with lower O2.
I've been in some pretty marginal atmospheres and feeling tired, irritable, headachey is one thing, CO2 is another.
In hindsight, I'd be tempted to run a Davy lamp/Lighter combo for Oxygen and get a dedicated CO2 meter.
A friend and I were surveying a small mine the other day and disturbed a layer of CO2 resulting in about a 6% atmosphere of it. We both nearly spat our lungs out by breathing so hard. I would have serious reservations about going into a "woody" mine with limited ventilation in a very dry spell like this one (CO2 is not dissolved and builds up).
A davy lamp running kerosene will go out at 15%, this (from my experience is good for ME to abseil/prussic in) a butane lighter will not light at about 13% which is a good level to get out of on the flat. CO2, I'd set 2.5% as a total and utter maximum and run like hell (without disturbing the layer) out of there!
I've spoken to a few mine exploring veterans about this experience and it seems to be one of the biggest concerns gaswise.
With low 02, you can go very very slowly out, or up a rope (assuming you are still sensible) whereas CO2 can make you breathe to utter exhaustion.
The only thing in my estimation good about a 4 gas is that you can set the O2 alarm points so the bloody thing doesn't go off when someone farts.
Edit:- This assumes that you are not going into coal mines and you will be sometimes abseiling into totally unknown holes ventwise. If you aren't abbing into the unknown, I'd probably use myself as a CO2 meter (even though it's very frightening when "it" goes off)
Edit2:- As my chums and I have done, please find your own comfortable level with low oxygen and set realistic limits. Take it very slowly in bad air and if in doubt, get out. I couldn't find any information on the internet about what levels were really good to go at and my levels are purely related to my giant gorilla but probably quite fit otherwise physique!