What I know, or think I know:
1) sleeping bag ratings standards - not really any industry standard. Some companies obviously rate conservatively and some do not. Some appear to rate by loft and some by other methods. Best way to use ratings is by checking boards such as this and ones for camping/backpacking to see what others think of a particular manufacturer's rating and then factor in your own factors (warm sleeper, cold sleeper, etc).
2) Some manufacturers will tell you how they rate and why they use the formula they use. Most do not tell you either. I imagine Pan came up with his formula above by a combination of experience and knowledge about insulation. His comments always seem to be pragmatic and founded in logic... which is much appreciated, especially considering he is also a manufacturer.
3) Water column - the number, usually in millimeters, is the height of a column of water that the material will not allow to pass through the material. So, a 5000mm water column rating means the material can keep a column of water that is 5000mm (is that 5 meters?) high from passing through it. These ratings are used to convey waterproofness, as the higher the #, the more waterproof the material is. However, perspiration is not water - it is water vapor and can pass through much smaller spaces than can water. So a highly waterproof material may still be breathable. Used to be, the more waterproof the material, the less breathable. Now things are starting to even out with some technical materials being both waterproof and breathable... and often expensive.
Hope this helps.