If you want some generalities, in the Shetland fleece I've known, the loss has been closer to 30% -- after skirting. So if I had 10 pounds to start, skirted, I'd expect to end up with at least 7 pounds. (5 pounds raw, skirted should end up with about 3.5 pounds washed.)
Things that can go wrong --
- lots of VM in the fleece. Though VM isn't all that heavy, it can cause you to toss out extra clumps of fleece; I had a Shetland whether who for some reason always had a ton of hay stuck along his spine, so his skirted fleeces always looked like donuts! (he's the black one in the picture -- you can even see the hay there! bad boy sheep! LOL)
- not fully skirted (all the junk around the outer edges -- legs, belly, backside). These may be muddy, thus heavy, really adding to the weight of the fleece.
I usually expect, to be safe, that from raw to roving I'll lose about 40% of the weight. If it's low-grease and clean, I might lose only 20-25%. If it's high-grease and clean, I could still lose about 35%. But 9 times out of 10 I end up "re-skirting" almost every fleece to save myself some hassle (or broken drum carder teeth) in carding. Shetland's typically a "mid-grease" fleece, in my experience.
(posted by me on Spin-List, this day)