Fill power doesn't measure crush resistance. It measures the inverse of density, in units of cubic centimeters per gram or cubic inches per ounce.
Insulation doesn't depend on fill power. Insulation simply depends on thickness. However, a higher fill power allows you to achieve a greater thickness while carrying a given weight on your back.
Fill power is measured when the down is puffed up, not compressed. When it's compressed, it gives very little insulation. For this reason, the part of your sleeping bag underneath you is extremely ineffective at keeping you warm. This is why people use sleeping pads, and some people use down blankets instead of sleeping bags.
Because insulation depends so strongly on the amount of compression, it's not possible to say in general that x centimeters of (uncompressed) down will provide y amount of insulation. For example, a down jacket will not be compressed as much as the bottom of a down sleeping bag in normal use.
It also depends on the wind. In a strong wind, there is a certain amount of difference in comfort between a thick layer of down and no layer of down. In totally still air, this difference is much smaller. Essentially all the down does is to provide you with a still layer of air, preventing convection. If the air is completely still anyway, you don't need the down for this.