Probably closer to 20% - and that doesn't include the (possibly) extended tail. I'm not sure how back the original construction went, although by the time the new bigger engines came along (or shortly thereafter) it only went back about as far as it does now. But given that a snow train plowing west from Alamosa would still be facing the "right" direction to plow out the tail, it wouldn't make a lot of sense to build it back very much further than it is now. They'd mainly want to be protecting the switch.
But back to lengths and stuff looking at Google Earth we see:
In the photo the existing portion of the snowshed is about 100 feet long and the original (assuming it started / ended about where it does now) was about 500 feet.
If you look closely, just to the right (east) of highway 17 and below the tracks you can see a small area that I've highlighted to show the markers we placed to note the original location of the depot. I've also highlighted the area where we believe the turntable was originally located (near the end of the yellow measurement line I drew. If you look closely you will see an arch that marks where we believe the turntable wall was located (we did some digging looking for timbers that might have been used as a foundation for a turntable). When the K class engines started showing up they removed the turntable and realigned the track to it's current position. Also, if you look closely, you can see a small area north of and across the tracks from the section house, also highlighted, where the two original water tanks stood (just to the east of the current standpipe). Lastly I've highlighted the area where the Lively gas station and "motel" once stood.
Just to give some perspective on the whole layout and the way it was.
Don