I don't know that there IS some standard number for applying to a down sample.
1) Static electricity
3) Age of sample
4) Type of cleaning, (wether cleaned at all)
5) Bleaching agents used for cleaning (bleach will remove plume barbs)
6) Organics left over from processing
7) Differences between samples
and so on...
Each item will add maybe a 20 ci difference, with some adding more than others. Some are spec'ed to maintain controll's and others are not. There are more(turbidity and oxygenation...likely beyond scope of a home tester.) Higher qualty down fills will compress MORE than low quality fills, hence the importance of using a standard sized glass tube. The methode described with your bear vault would be good, except the flotation area will change vs a standard test. The second thing is that you need two numbers, generally 24 hours apart, though there is some tollerance of unknown time. Example: 12+ hours could be rounded to a day...12- hours could not.
A lot of things go on to achieve the best numbers and worst numbers depending on who is doing the testing. Off hand, I would say you are lucky to get within 200ci of a standard test.
Usually, comforters with "down" fill as opposed to "feather" fill are around 450-550. Old pillows are usually "feather" fill which means 50% of the fill is chopped or unchopped feathers. Feathers have a "stalk". Down does not.
So, IFF you stick to the standards, you *might* come within 100fp. Using your bear ball without standard drying/washing/humidity you will be within 200-300fp.
Anyway, yes, a maximum lofting would be achieved with no weight. Fill power and loft generally apply to the same thing. Example: 600FP (Fill Power) down will give you a 2" loft. FP is a cubic measurment of volume. Loft is the result of X amount of Y FP down in a known space, Z or roughly a linear measument. Not to worry, we know what you mean with either term by the context. (I HATE english, it lets this happen.)
The terms "down" and "feather" are often misused. The US simply say the majority. So, >50% is DOWN, and <50% is FEATHER. But, I am not sure what they consider "down." I do not consider anything with a stalk, down. Even the tiny little feathers that will occasionally pop through a bag. They *might* consider the downy bottom of a feather down. A BIG difference.
So, often this is where integrity comes in. I would trust the measurments done by WM, FF, or Nunatak. I would not trust Coleman, or some others that "use" down without testing. Producers rarely test more than one or two samples. More if they have integrity, meaning their supplies are likely to be more varied, less so if they really consider feathers/down a byproduct.