1.2V represents the static voltage of a NiCad and or NiMH cell.

Multiply the cell count by 1.2 for the static voltage rating of the battery pack.

Multiply the cell count by 1.1 under a load for a safe no fly voltage, in reality you can go lower. Possibly 1.0V per cell but I'd don't think it's prudent. Todays cells fall of the discharge curve around 2/3 the rated capacity at 2C and 1.1V. This is much better than cells of previous years.

A safe no fly voltage for 4.8V batteries is 4.4V and 5.5 for 6V.

One has to remember or realize that these cells reach a plateau on the discharge curve and maintain this level of volatge for the bulk of the discharge cycle and then volatge and current falls off sharply, thereafter.

There are several locations to view discharge curves of many cell types to validate my claims or you can simply do a load test on one of the many battery charger/cycler's to verify your batteries capacity and average voltage under load against consumed mAh.

However the fact of the matter is relying on a loaded voltage to determine the safe NO FLY voltage is unreliable and hap hazard IMO. The best method is to evaluate your batteries by cycling them, charge your batteries and fly your model for a measured time (say 30 minutes). Cycle your batteries and record the mAh capacity left after the 30 minute period. Subtract the consumed mAh capacity from the known cycled capacity, use this data with the 30 minutes time frame to determine the mA consumed per minute and weigh this against the battery capacity to estimate the amount of time you can fly. Use 60% of the capacity of the cycled battery as safe no fly zone calculation.

A typical 33% model (9 digitals) consumes around 22mA per minute under normal flight routines. If I have a single 1600mAh battery that cycles at rating, 1600mAh and 60% of 1600 = 960mAh. 30 minutes times 22mA = 660mA. As you can see this provides a much more accurate method of actually knowing when to stop flying. Together with this info and the loaded voltage reading of your model you can become familiar with your models system and typical numbers at a given time period of flying. It's easy to spot problems as you become more familiar with the loaded voltage and time flown correlation.