Oregon Shepard gives out free samples. I would say, get the free sample, then make a small pouch to stuff it in. Your warmth rating will be based on the loft, and is probably comparable to loft of down. Then you weigh it and determine how much for a certain amount of loft. Another method would be to simulate the down rating test. I believe it uses a column of down with a weight.

edit: There is a chart on their site that give R value, thickness and weight together. A little math is all that is needed.