Well, You could try the suggestion.
Older down was not really well graded. Often it was a mix of down feathers and down plumes. Volume testing, as was mentioned, works fine. But, if the items were that heavily used, I suspect the down will be dirty. This will effect the volume a lot.
Static charges will be much reduced, oils may cause the plumes to be less full than what a factory will measure. Moisture will have the same effect. Soo, be a bit cautious about the final numbers you get.
Typically, the down will be washed a couple times. Then fluffed as it is dried. Then it is measured. When you get a 2oz bag of 800 fill down, it will likely measure less than a standard measurement. It needs to be heated to about 200F and fluffed simultaneously. (Don't try this with a hair dryer...it means down all over! It is really messy.) This will give it the maximum volume.
By visual inspection, if there is a lot of recognizable feathers, it is 500 to
600 fill. If there are any chopped feathers, it is less than 500. If there is some feathers, between 600-700. If there are no recognizable feathers, 700-800.
By EN measurements, 800 fp is as good as it gets. Then it starts over with the grade of down plume. A fully mature Eider Duck plucks fully mature down plumes from her belly/under wings to line her nest. The Chineese slaughter geese and pluck everything. Guess which is better? Eider down is considered as 900fp even though it never acheives this. It is just a better insulator. Goose down is not real bad. it can get to 830fp depending on the year. Anyway, this doesn't help you with grading.
Typically, most sleeping bags are overrated by about 100 points, since the weight of the shells compresses the down. Hence the temperature ratings of the EN measument scales. Hard to get an 800fp down to NOT compress with a simple breath of air. Soo, they put in 800, but, we never get that. But, it is what they advertise. Using what you have, I think I would simply visually inspect it, and let it go at that.
Larger down plumes are generally better. But, looking at stiffness is important, too. An immature plume will not have as many barbules off of it. It is hard to grade without a lot of experience... I don't have that expierence, either. I have enough info to know it is a difficult thing to do.
I would simply use what you have. Don't waste it. Get some spinnaker cloth and make a nice vest. I don't think I would want to trust sleeping in it, unless you have a LOT. It could go either way. Not that great. Or just warm and toasty. For a few yards of fabric, it is well worth the effort, IMHO.