I just read many posts on measuring DE and using DE scoops, most from 2012 and 2014 and have some comments.
1) I believe the dry bulk density of 20 lbs/cu.ft. cited by JamesW [and is in an EP Minerals data sheet, attached] is an "unpacked" density. So if you load your scoop and pack it to cram as much DE into the filter as possible, you may be loading too much DE. Whether it is critical or not depends on how much the DE can be compressed, how many backwashes you do before cleaning, and how well you slurry the DE into the skimmer. (I would guess packed, unslurried DE might lead to very uneven coatings)
2) Not all scoops have the same volume. I saw 48 ounces cited a few times, 24 ounces once, and several comments about how much weight a person's scoop holds without giving the volume. I bought a scoop last fall and called the store a few days ago when I realized it had not markings. I was told it was probably 2 cups..."which would be a pound". Turns out is is 4 cups plus 4 ounces which is, at the 20 lbs/cu.ft. density, is .75 pounds.
3) As JamesW said in a few posts, the 20 lbs/cu.ft. equates closely to 48 fluid ounces per pound. So unless you like to use a scale, it seems the best way to measure out the DE is multiply the number of pounds you need by 48 fl.oz./lb and divide by the number of fluid ounces your scoop holds. And don't pack it.
[The opinion on not packing is partly based on what I learned in an engineering course on the properties and handling of powders on an industrial scale. However that was over 30 years ago and I have forgotten over 90% of what I learned so feel free to disagree].