Roger,
It think most of the Lipoly monitoring devices out there indicate when the cells reach 3.0v-2.8v per cell. This way it gives you time to get the plane on the ground without damaging the cell. I don't use a monitor myself as one of the features of Lipolys is that they have a very flat discharge rate until there almost done then there is a rapid drop in voltage. It is fairly noticable that you now have to carry full throttle to maintain level flight so thats when you land. I have yet to damage a cell by discharging it too much. The only exception to this rule of thumb might be if the plane is able to fly at a very low throttle setting (such as a floater type plane) you may not notice the decrease in performance.
Just my 2 cents worth.
Regards,
Dan Hurd
DWE
www.smallrc.com